Now, let's move on to the important concepts and formulae related to JEE and JEE Main exams along with a few solved examples.

Today, most beginner telescopes of any quality are sold with either Kellner or Plossl eyepieces. Those that still come with Huygens and Ramsdens are generally cheap and low quality, since practically the only reason to use those designs in modern times is that they are extremely cheap to manufacture. Very often, those eyepieces will be in the outdated 0.965” format rather than the modern 1.25” or 2” diameter barrel format.

Optics is like the magic of light and how it interacts with the world. It helps us understand how we see things, how lenses work, and how rainbows form. Imagine it as the science of understanding light and its tricks, like a magician revealing the secrets behind the illusions. In your JEE studies, optics is an essential topic that helps you see the world through the eyes of science.

It is given that the rays are falling normally on face AB. If the angle of incidence is greater than C, then the rays will suffer total internal reflection. Thus, if $\mu$ is greater than 1.414 then the colour will suffer total internal reflection.

I have seen some gatekeeping in this hobby about Kellners, and I’m not sure I understand it. People may have heard, and rightly so, that Huygens or Ramsdens are crummy eyepieces and that they should get a Plossl instead, and they may believe the same applies to Kellners, but the truth is that Kellners are often good eyepieces. Yes, the wider field of view of a Plossl is very helpful, but 40 degrees of Kellners isn’t a soda straw (unless you are used to looking through very wide field eyepiece. Oh the pain of… uh… having access to really cool eyepieces.)

2.A beam of light consisting of red, green and blue colours is incident on a right angled prism, as shown in figure. The refractive indices of the material of the prism for the above red, green and blue wavelengths are 1.39, 1.44 and 1.47, respectively. The prism will

Every particle on the given wavefront acts as a fresh source of new disturbance, called secondary wavelets, which travels in all directions with the velocity of light in the medium.

A prism is a triangular optical element that refracts light, causing it to deviate. The deviation is different for different colors due to dispersion, and this phenomenon is key in understanding how rainbows form and how prisms split white light into its constituent colors.

In 1849, Carl Kellner invented the Kellner eyepiece, which is essentially a Ramsden eyepiece where one lens has been replaced with an achromatic doublet made of two different types of glass. The three-lens eyepiece that came out of this was much better at controlling false color fringing and other distortions, even in telescopes with short focal lengths. By correcting for aberrations, the field of view of these eyepieces could be much larger than the “soda-straw” field of view in a Ramsden or Huygens, around 40 or perhaps up to 50 degrees. The tradeoff was that each lens surface added another internal reflection, reducing contrast and adding glare.

The mirror formula is a relationship between the mirror's focal length and the distance between the object and image from the mirror.

Key point: The concept of total internal reflection and the relation between refractive index and wavelength of light are the important keys to solve this problem.

1. When a glass plate with a refractive index of 1.3 is put in the path of one of the beams in Young's double slit experiment, the fringes are displaced by a distance $x$. The fringe shift is $(5/2)x$ when this plate is replaced by another plate of the same thickness. The second plate's refractive index will be_______.

A simple microscope is used for observing magnified images of tiny objects and it consists of a converging lens of small focal length.

Optical

Plossl eyepieces have an eye relief equal to about 80% of their focal length. This is perfect for a 25mm Plossl (an extremely common standard) and downright luxurious with a 32mm Plossl, but when you get below about 12.5mm or 10mm, it can begin to get difficult to use.

Explore an array of resources in the JEE Main Physics Study and Practice Materials section. Our practice materials offer a wide variety of questions, comprehensive solutions, and a realistic test experience to elevate your preparation for the JEE Main exam. These tools are indispensable for self-assessment, boosting confidence, and refining problem-solving abilities, guaranteeing your readiness for the test. Explore the links below to enrich your Physics preparation.

Key point: The concept of interference of light and its conditions for bright fringes is important to solve this problem.

When a beginner is choosing eyepieces on a budget, there are a number of possible designs they may choose between. While eyepieces with relatively complex, proprietary, or bespoke designs are becoming more commonly available in the beginner price bracket, there is often good reason to choose a much simpler design.

Reflecting type of telescope consists of a parabolic concave reflector with a narrow hole at the centre and secondary convex mirror.

The phenomenon of total internal reflection is the reflection of light into a denser medium from an interface of this denser medium and a rarer medium.

Eye relief is the distance from the eye lens to your eyeball, and the higher the better (within reason). Eye relief of less than 10mm gets really uncomfortable—you tend to rub your eyepieces against the eyepiece or lens, and you have to hold your eye at just the right distance to keep from touching the lens. A long eye relief is also important for those who wear eyeglasses at the eyepiece, whether that’s a member of the general public at a star party or an astronomer with astigmatism, which, unlike near- or far-sightedness, cannot be corrected with the telescope’s own focuser.

Therefore, the wavelengths of yellow, orange and red colour light are greater than the wavelength of green colour light, while the wavelengths of violet, indigo and blue are lesser than green colour light.

The resolving power of optical instruments like microscopes and telescopes is a critical factor. It determines their ability to distinguish two closely spaced objects. The resolving power of a microscope or a telescope can be defined as:

1. In a single slit diffraction experiment, the first minimum for red light (660 nm) coincides with the first maximum for some other wavelength $\lambda$. Calculate $\lambda$.

For red colour, the refractive index is 1.39 which is less than 1.414 but for the green and blue colurs, the refractive indices are 1.44 and 1.47, which are more than the 1.414. Hence, red colour will emerge out of the prism while green and blue colours suffer total internal reflection. Hence, option a is correct.

2. The slits in Young's double-slit experiment are spaced by 0.3 mm, and the screen is 130 cm distant. To obtain interference fringes on the screen, a laser beam with two wavelengths, 600 nm and 500 nm, is utilised. Calculate the shortest distance between the shared centre maxima and the point where the bright fringes caused by both wavelengths coincide.

Finally, a wider field of view just makes it easier to find stuff. This is why we recommend you try to locate the object first at low power, then use a higher magnification if you like—the wider field of view on the sky provides more context in terms of asterisms and reference stars and makes it more likely that you’ll catch the object you are looking for.

Plossl and Kellner eyepieces are two very common examples of a simple eyepiece design, and they are probably the most common type of eyepiece you are likely to come across when eyepieces have been bundled with a telescope; they are also a very common aftermarket accessory.

A common problem are “eyepiece kits,” which are big boxes that some telescope suppliers will sell you with half a dozen eyepieces, most of which include annoyingly short focal length Plossl eyepieces. A 6mm Plossl has a tiny eye lens. I once had to use a 6mm Plossl to attempt to perform a star-test collimation for a telescope at the observatory. It was grueling, because I couldn’t even get the entire field of view—I had to keep my eye hovering above the exit pupil.

Diffraction is a phenomenon where light waves bend around obstacles and spread out. When light passes through a single slit, it diffracts, leading to the formation of a diffraction pattern. The width of the central maximum in a single-slit diffraction pattern can be determined using the formula:

Plossl eyepieces have wider fields of view than Kellners. By default, expect a 40-degree field of view in a Kellner and a 50-52 degree field of view in a Plossl. I have seen wider-field Plossls. There was an old Meade “Super-Plossl” design that used an additional lens element and could reach a wider field of view. I have a custom-built 32mm Plossl that has been opened up as wide as possible to 60 or 70 degrees, for use as a 2” eyepiece, although in my 10” Dobsonian at f/5 the edge distortions prevent it from seeing much use. There’s a reason Plossls are usually kept at 52 degrees.

Generally, to maximize the number of stars you see in an eyepiece with a given telescope, both the magnification and field of view should be maximized. Between Kellners and Plossls, this means a 32mm Plossl, which maximizes the field of view in the 1.25” barrel diameter format, will show more stars than either a 32mm Kellner with the same magnification but a smaller true field of view, or a 40mm Kellner (or 40mm 1.25” Plossl, which has a 40 degree apparent field of view) with a lower magnification and the same true field of view. Through the 32mm eyepiece, objects will look larger, and though they may have a lower surface brightness, the surface brightness of the background sky will also be lower.

By the 1980s, amateur astronomy equipment suppliers were still selling new telescopes with Huygens and Ramsden eyepieces, not merely because they were cheap but because they lacked internal reflections. You might be able to find a Kellner, or perhaps a high-quality Orthoscopic. As new anti-reflection coatings were developed, it became possible to actually make use of complex lens arrangements (not just in relatively simple 4-element systems but also in convoluted, extreme wide-field eyepieces such as the TeleVue Nagler).

Modern optics: Modern optics is a branch of optics that deals with the interaction of light with matter at the atomic and subatomic level. It includes topics such as lasers, fiber optics, and holography.

1. A beam of white light is incident on the glass air interface from glass to air such that green light just suffers total internal reflection. The colours of the light which will come out to air are

Combination of Thin Lenses in Contact: When two or more thin lenses are in contact, they can be treated as a single lens. The power of the combination is the sum of the individual powers of the lenses.

Polarization is the phenomenon where light waves vibrate in a specific direction. Brewster's Law helps us understand the angle at which polarized light is reflected from a surface. It is expressed as:

Here, you'll find a comprehensive collection of study resources for Optics designed to help you excel in your JEE Main preparation. These materials cover various topics, providing you with a range of valuable content to support your studies. Simply click on the links below to access the study materials of Optics and enhance your preparation for this challenging exam.

Power of a lens ($P$) is defined as the ability of the lens to converge a beam of light falling on the lens and it is measured as

The process of redistribution of light energy in a medium caused by the superposition of light waves from two coherent sources is known as interference of light.

In the early days of telescopic astronomy, refractor telescopes were designed with a single lens at the front and a single lens at the back. Centuries of improvements led to achromatic refractors, where the primary lens is made up of two lenses of different types of glass, which reduces false color fringing and allows the telescope to be made considerably more compact. In addition, the eyepiece would be a Huygens or Ramsden design, where two lenses made of the same type of glass could be combined to yield minimal false color fringing, as long as the focal length of the telescope remained fairly long.

With modern multi-coated optics that thwart internal reflections, Plossls and Kellners have little difference in glare or transmission/opacity.

Magnifying power ($m$) of a simple microscope is defined as the ratio of angles subtended by the image and the object on the eye, when both are at the least distance of distinct vision($d$) from the eye. It is given as

Wave optics: Wave optics is a physical approach to understanding the propagation of light. It is based on the principle that light is a wave, and that it can interfere and diffract.

The path difference of $\lambda$ has introduced the phase difference of $\beta$. Therefore, a path difference of $(\mu-1)t$ that will introduce a shift of $x$ is given as

At this point, a Plossl and a Kellner are so close in price and performance that you may as well get the Plossl. If you wish to find a short-focal length eyepiece with a longer, more comfortable eye relief, you could use a long-focus eyepiece with a separate, high-quality barlow lens (an achromatic or apochromatic fully multi-coated barlow would be necessary), or you could purchase one of the several designs incorporating a smyth/barlow lens into the eyepiece design. These are effectively simple designs like Plossls, Kellners, Konigs, and RKEs, with long focal lengths, that have a lens included to increase their focal length. Within the budget-friendly realm are the 6mm and 9mm Goldlines/Redlines (66-degree Ultra-Wide-Angle) for telescopes with a focal ratio of f/8 or longer, and the excellent 58-degree Planetary eyepieces, sold by a variety of sellers on Amazon and by Agena Astro, which come in a wide variety of focal lengths and are optically superb for their price.

The phenomenon of restricting the vibration of light in a particular direction, perpendicular to the direction of the wave motion, is called polarisation of light.

To solve this problem, we have to use the relation between the thickness of a plate, path difference and its refractive index. Also, we have to use the relation of fringe width.

optics期刊

From the above relation, we can conclude that the central maxima of $5^{th}$ bright fringe of $\lambda_1$ coincides with the $6^{th}$ bright fringe of $\lambda_2$.

Optica

Plossls and Kellners, like other simple eyepiece designs, have moderate fields of view and tend to have short eye relief as you scale them down to shorter focal lengths.

A lens is a section of a transparent refracting material that is joined by two spherical surfaces or one spherical surface and one plane surface. There are types of lenses:

Another useful aspect of a wider field of view is that, assuming the entire eyepiece is sharp enough to show good images right to the edge, you don’t have to nudge the telescope to track the rotation of the Earth as often with a Plossl (50 degrees) compared to a Kellner (40 degrees). The difference isn’t that severe, and I often track more often than is strictly necessary to keep the object centered anyway.

Ray optics: Ray optics is a geometrical approach to understanding the propagation of light. It is based on the principle that light travels in straight lines, and that it can be reflected and refracted at interfaces between different materials.

Waveoptics

Hence, $C$ will be greater for yellow, orange and red colour light. Therefore, these colour lights will come into the air from the glass.

Here, $n$ = 1,2,3……an integer, $a$ = width of the aperture, $\theta$ = angle made by secondary waves while travelling and $\lambda$ = wavelength of light.

AMOphysics

Astronomical telescope consists of two lenses i.e, an objective lens which is of large focal length($f_o$) and an eyepiece which has a small focal length($f_e$).

Even though both eyepieces are very common in beginner telescopes, it has become harder to find Kellner eyepieces on their own in the last two years, whether they come from a brand name or the original equipment manufacturer or a reseller. Meanwhile, Plossl eyepieces are much more common and easy to find. As of the past couple years, it seems possible that the choice between Kellners and Plossls has been made for us—you either get what you get with your beginner telescope, or you will have to find a Plossl.

When I was getting my start in the hobby, I bought an ultra-cheap ($15?) 6mm Kellner eyepiece that I enjoyed a lot. Despite the narrow field of view and short eye relief, it showed shockingly good views of Saturn, Jupiter, and even the Moon. Even when I upgraded to a 6mm Goldline, I kept the Kellner around because of pretty annoying kidney-bean blackout issues on the Moon with that eyepiece. I foolishly destroyed it while experimenting with solar projection, because I hadn’t realized the metal-and-glass construction was aided by a plastic retaining ring on the field stop, and I hadn’t realized you do not need to change the eyepiece focal length when doing solar projection. Generally, don’t mess around with solar projection unless you’re okay with damaging the instrument. And of course, NEVER look through a telescope pointed at the sun that does not have a safe solar filter covering the front.

Optics manufacturers began selling and popularizing Plossl eyepieces, albeit with a generally simplified design where each achromatic doublet had the same focal length. These eyepieces are technically called Symmetrics, since Plossls can have different focal lengths for each doublet, but they are always marketed as Plossl.

In order to solve this problem, we have to apply the concept of central maxima in interference pattern. Let $x$ be the distance between the centre maxima and the point at which the bright fringes caused by both wavelengths coincide.

According to the Law of Malus, when a beam of completely polarised light is incident on an analyser, the resultant intensity of light ($I$) transmitted from the analyser varies directly as the square of the cosine of the angle ($\theta$) between plane of transmission of analyser and polariser.

Optics

Kellners tend to have narrower apparent fields of view (40 degrees rather than 50 or 52 degrees in a Plossl), and in short focal length telescopes (focal ratio f/5 or shorter) they can have more false color fringing, but for the most part it is hard to notice the difference between a well-made Kellner and a well-made Plossl in terms of image quality. Many Kellners are made with field stops opened up to 50 degrees, in which case it can truly compare to a Plossl in any telescope with a focal ratio of f/6 or slower.

optics中文

In this article, we'll dive into the fascinating world of optics, a crucial chapter in JEE Main physics. We'll explore the fundamental concepts and tackle questions related to optics. Discover the secrets behind the behavior of light, reflection, refraction, and the wonders of lenses and mirrors. You'll find all you need in one convenient place. With free downloadable PDFs, students can easily grasp these concepts and problem-solving techniques. This resource will be a valuable asset for your JEE Main preparation, helping you excel in your exams.

2. Find the minimum thickness of a film which will strongly reflect the light of wavelength 589 nm. The refractive index of the material of the film is 1.25.

The dispersive power of a prism is solely determined by the type of prism material. However, angular dispersion and mean deviation, both depend on angle of prism in addition to the nature of material of the prism.

Lens maker’s formula is a relation that connects focal length($f$) of the lens to radii of curvature ($R_1$ and $R_2$) of the two surfaces of the lens and refractive index($\mu$) of the material of the lens. It is written as

Despite one being fundamentally superior to the other in terms of image correction, Kellners and Plossls are generally quite comparable in terms of the image you actually see.

Optics is perhaps one of the most often questioned topics in JEE main, particularly in JEE Advanced. So, certainly, it is heavily weighted in the JEE examinations. Every year, approximately 3-4 questions are asked from this chapter, which leads to approximately 4-5 % weightage.

History ofoptics

For interference to occur, the sources of light must be coherent, meaning they have a constant phase relationship. Coherent sources are essential for sustained interference patterns. These patterns play a crucial role in applications such as interferometry and the study of thin films.

I think some of these kits even include 4mm eyepieces, and that’s ridiculous! Even if you did need a 4mm focal length (for high powers in an f/4 telescope, of course), neither a Kellner nor a Plossl would have long enough eye relief for that to be comfortably usable! You’d have to look into more complex designs which include a barlow lens, or simply use a longer focal length eyepiece with a quality barlow.

The phenomenon of light dispersion is the splitting of a white light beam into its constituent colours when it passes through a prism.

In 1860, Georg Simon Plossl invented the Plossl eyepiece. A four-element eyepiece was made by putting together two achromatic doublets with different focal lengths. This eyepiece was even better at correcting false color fringing, but because of internal reflections, the glare was even worse.

The field of view of the eyepiece has a few effects when it comes to actually using it in the telescope. For one thing, a wider field of view is almost always more pleasant. You see the object in a wider expanse of space with more stars. In some cases, an object is so large (i.e., the Moon, the Pleiades, the Orion Nebula, the Andromeda Galaxy) that a narrow field of view can only show you part of the object, whereas a wider field of view could show the whole object at the same magnification.

Kellners are often sold with cheaper, even more entry-level telescopes than Plossls. Plossls tend to come with large, ground-standing Dobsonians, whereas you’re more likely to find a Kellner sold with a tabletop Dobsonian. Most of Celestron’s entry-level telescopes are sold with Kellners (and other cheap but reasonable-quality designs like RKEs or Konigs), whereas their more expensive SCTs tend to come with a single 25mm Plossl.

You probably don’t need to worry about upgrading them as soon as you get your telescope, unless you can see an optical problem in the one you’re using or you find the short eye relief for the short-focus eyepiece uncomfortable. If you do upgrade, you’ll probably want to go for a different eyepiece design than a Plossl–something with a sharper image or a wider field of view or a longer eye relief (or all three.)

The relation between the path difference($\Delta$), the thickness of the plate ($t$) and its refractive index ($\mu$) is given as

According to the question, it is given that white light is incident on the glass air interface from glass to air and green light suffers total internal reflection. We know that white light consists of seven bands of colours i.e, VIBGYOR whose wavelength increases as we go from violet to red colour.

If someone wants to score well and achieve a better rank in JEE competitive exams, then it is very important to prepare for this chapter. Moreover, the weightage of this chapter is more as compared to the others; hence it has to be given more importance.

Here, $f_o$= focal length of objective lens, $f_e$= focal length of the eye-piece, $L$= length of the microscope tube and $d$= least distance of distinct vision.

Compound microscope consists of two lenses co-axially at the free end of a tube, at a suitable fixed distance from each other.

(Focal ratio is a term that refers to the telescope’s focal length divided by its aperture. Higher values are slower and lower values are faster–this is a holdover from camera lens terminology and has to do with exposure times.)

We must practise previous years’ questions in order to score well and become familiar with the exam's difficulty level. It not only enhances our self-esteem but also exposes us to areas where we may need practice. Solving previous 10 to 15 years’ question papers can help you better comprehend a concept and can also give you an indication of how many times a concept or topic has been repeated in the test. You can prepare for physics chapters and measure JEE notes by practising previous years’ questions, which will turn out to be beneficial.

Kellners can be as little as half the price of a Plossl in the same focal length, but they are generally very comparable, especially in recent years as telescope prices have fluctuated in response to changing supply chains and demand. Comparing Orion Telescope & Binoculars’ options, it appears that a Kellner can be found for almost half the price of a Plossl of the same focal length, although there are also equally cheap Plossls available from various sellers on Amazon that are highly comparable. (Svbony’s Plossls, for example, are optically identical to Orion’s Sirius Plossls).

At any instant, a surface encountering these secondary wavelets tangentially in the forward direction yields the new wavefront at that instant. This is referred to as a secondary wave front.

I have several 10mm eyepieces in my eyepiece box—almost all of them are eyepieces that came with some telescope I bought—and I will almost always choose to use one of the Kellners over one of the Plossls, because the Kellners have a longer eye relief, which is comparable to the focal length. When I had a 6mm Kellner, I found the eye relief annoyingly short, sure, but when compared to the 6mm Plossl that often comes with “eyepiece kits,” it was luxurious.