Depth of field - what is depth of focus

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Directly below is an illustration of a hydrogen emission spectrum. The spectrum is a solid black rectangle with four colored vertical lines. From left to right, the colors are purple, blueish-purple, blue, and red. The lines are not evenly spaced or even in width.

The three graphics on the right side of the infographic are aligned to show the relationship between the color of light absorbed and the electron jumps, the absorption lines in the picture of the spectrum, and the absorption valleys on the graph.

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A diagram of a hydrogen atom shows the relationship between the color of light absorbed by an electron and its change in energy level.

Your ophthalmologist will do an eye exam to look for warning signs of ischemic optic neuropathy (ION). They will dilate (widen) your pupils with eye drops and then check for swelling of the optic nerve and blood vessels in the back of your eye.

ION can affect your central (detail) vision, side (peripheral) vision, or both. Because a damaged optic nerve cannot be fixed, any vision loss from ION is usually permanent. Usually, people with severe ION still have some peripheral vision.

There is no treatment to improve vision loss from ION. However, your ophthalmologist may suggest magnifiers, assistive technology devices and other techniques to manage your activities with low vision. Fortunately, ION happens in one eye more often than in both eyes.

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Four scenarios involving absorption of light and electron jumps are shown. In all four cases, the electron is represented as a small circle. Light is represented as a wavy colored arrow. The change in energy level is shown with a dashed, straight white arrow. In all four scenarios, the small circle is positioned on energy level 2 to indicate the electron’s starting energy level.

A diagram of a hydrogen atom shows the relationship between the change in an electron’s energy level and the color of light emitted by the electron.

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If blood flow to your optic nerve is reduced, your vision will darken for a few seconds or minutes then return to normal. This is called a transient ischemic attack (TIA). This kind of attack can happen before ION begins. If you have TIA symptoms, call your ophthalmologist or primary care doctor right away. Finding and treating the problem as soon as possible can help prevent further vision loss from ION.

Molecules, like water, carbon dioxide, and methane, also have distinct spectra. Although it gets a bit more complicated, the basic idea is the same. Molecules can absorb specific bands of light, corresponding to discrete changes in energy. In the case of molecules, these changes in energy can be related to electron jumps, but can also be related to rotations and vibrations of the molecules.

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This four-part infographic titled “Emission of Light by Hydrogen” illustrates the relationship between the change in energy level of an electron in a hydrogen atom, the wavelength of light emitted by the atom, the emission lines in the hydrogen spectrum, and the graph of hydrogen’s emission spectrum. The graphic includes:

If your ION is caused by swelling of arteries in your head (temporal arteritis), your ophthalmologist may have you take steroid (prednisone) pills. This medicine may prevent ION from developing in your other eye.

Ischemic optic neuropathy (ION) is when blood does not flow properly to your eye’s optic nerve, eventually causing lasting damage to this nerve. With ION, you suddenly lose your vision in one or both of your eyes. The optic nerve carries signals from your eyes to the brain. Your brain then turns these signals into the images you see. When blood flow to the optic nerve is reduced or blocked, the nerve does not get enough oxygen or nutrition. The optic nerve stops working properly, and eventually dies. ION can affect your central (detail) vision, side (peripheral) vision, or both. Because a damaged optic nerve cannot be fixed, any vision loss from ION is usually permanent. Usually, people with severe ION still have some peripheral vision. What are symptoms of ION? If blood flow to your optic nerve is reduced, your vision will darken for a few seconds or minutes then return to normal. This is called a transient ischemic attack (TIA). This kind of attack can happen before ION begins. If you have TIA symptoms, call your ophthalmologist or primary care doctor right away. Finding and treating the problem as soon as possible can help prevent further vision loss from ION. Who is at risk for ION? While anyone can get ischemic optic neuropathy (ION), you are more likely to develop it if you: have high blood pressure have high cholesterol (too much fat or lipid in your blood) sleep apnea heart disease sudden drop in blood pressure or blood loss have diabetes have clogged arteries have glaucoma have migraine headaches have swelling of arteries in the head (called temporal arteritis), which may be life-threatening and cause massive vision loss People 50 and older are also more likely to get ION. Diagnosis Your ophthalmologist will do an eye exam to look for warning signs of ischemic optic neuropathy (ION). They will dilate (widen) your pupils with eye drops and then check for swelling of the optic nerve and blood vessels in the back of your eye. Your ophthalmologist may also: test your side (peripheral) vision and measure the fluid pressure within your eye. do a blood test to look for signs of giant cell arteritis (also known as temporal arteritis or inflammation of blood vessels) Treatment If your ION is caused by swelling of arteries in your head (temporal arteritis), your ophthalmologist may have you take steroid (prednisone) pills. This medicine may prevent ION from developing in your other eye. Your doctor may want to treat any other health problems you have that put you at risk for ION. They may prescribe medicine for high blood pressure, diabetes, clogged arteries, migraine headaches, or other health problems. There is no treatment to improve vision loss from ION. However, your ophthalmologist may suggest magnifiers, assistive technology devices and other techniques to manage your activities with low vision. Fortunately, ION happens in one eye more often than in both eyes.

At the center is a solid circle representing hydrogen’s nucleus. Six concentric circles representing electron energy levels (or orbitals) surround the nucleus. The circles are labeled “level 1” through “level 6” with level 1 closest to the nucleus, and level 6 farthest. The distance between adjacent energy levels decreases with distance from the nucleus.

At the bottom, directly below the picture of the spectrum is a graph of the same spectrum. The vertical y-axis is labeled “Brightness” with labeled tick marks ranging from 0 at the bottom to 0.6 at the top, in even increments of 0.1. The horizontal x-axis is labeled “Wavelength (nanometers)” and ranges from about 375 nanometers at the origin on the far left, to about 775 nanometers on the far right. The axis is labeled in even increments of 100 nanometers, starting at 400 nanometers.

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This four-part infographic titled “Absorption of Light by Hydrogen” illustrates the relationship between the wavelength of light absorbed by an electron in a hydrogen atom, the change in energy level of the electron, a picture of the absorption lines in the hydrogen spectrum, and the graph of hydrogen’s absorption spectrum. The graphic includes:

Superimposed on the curve are absorption features: four steep valleys of relatively low brightness. From left to right, the valleys appear at wavelengths of 410 nanometers, 434 nanometers, 486 nanometers, and 656 nanometers. The depths of the valleys increase from left to right.

The interesting thing is that the electron can move only from one energy level to another. It can’t go partway between levels. In addition, it takes a very discrete amount of energy—no more, no less—to move the electron from one particular level to another.

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The optic nerve carries signals from your eyes to the brain. Your brain then turns these signals into the images you see. When blood flow to the optic nerve is reduced or blocked, the nerve does not get enough oxygen or nutrition. The optic nerve stops working properly, and eventually dies.

The absorption spectrum of hydrogen shows the results of this interaction. In the visible part of the spectrum, hydrogen absorbs light with wavelengths of 410 nm (violet), 434 nm (blue), 486 nm (blue-green), and 656 nm (red). Each of the absorption lines corresponds to a specific electron jump. The shortest wavelength/highest energy light (violet 410 nm) causes the electron to jump up four levels, while the longest wavelength/lowest energy light (red 656 nm) causes a jump of only one level.

Different elements have different spectra because they have different numbers of protons, and different numbers and arrangements of electrons. The differences in spectra reflect the differences in the amount of energy that the atoms absorb or give off when their electrons move between energy levels.

The spectrum is a straight flat line of very low brightness with four steep, sharp peaks of high brightness representing hydrogen’s emission features. From left to right, the peaks appear at wavelengths of 410 nanometers, 434 nanometers, 486 nanometers, and 656 nanometers. The heights of the peaks increase from left to right.

At the bottom, directly below the picture of the spectrum is a graph of the same spectrum. The vertical y-axis is labeled “Brightness.” The horizontal x-axis is labeled “Wavelength (nanometers)” and ranges from about 375 nanometers at the origin on the far left, to about 775 nanometers on the far right. The axis is labeled in even increments of 100 nanometers, starting at 400 nanometers.

Let’s go back to simple absorption and emission spectra. We can use a star’s absorption spectrum to figure out what elements it is made of based on the colors of light it absorbs. We can use a glowing nebula’s emission spectrum to figure out what gases it is made of based on the colors it emits. We can do both of these because each element has its own unique spectrum.

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Four scenarios involving emission of light and electron drops are shown. In all four cases, the electron is represented as a small transparent circle. Light is represented as a wavy colored arrow. The change in energy level is shown with a dashed, straight white arrow.

Your doctor may want to treat any other health problems you have that put you at risk for ION. They may prescribe medicine for high blood pressure, diabetes, clogged arteries, migraine headaches, or other health problems.

Electrons can also lose energy and drop down to lower energy levels. When an electron drops down between levels, it emits photons with the same amount of energy—the same wavelength—that it would need to absorb in order to move up between those same levels. This is why hydrogen’s emission spectrum is the inverse of its absorption spectrum, with emission lines at 410 nm (violet), 434 nm (blue), 486 nm (blue-green), and 656 nm (red). The highest energy and shortest wavelength light is given off by the electrons that fall the farthest.

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Directly below this graph is an illustration of a hydrogen absorption spectrum. The spectrum is a rectangle with rainbow coloring: purple on the left to red on the right. The rainbow pattern is not continuous and includes four black lines of varying width.

Four scenarios involving electron drops and emission of light are shown. This graph is almost identical to the diagram of the hydrogen atom on the left side of the infographic, but does not show the nucleus of the atom. The energy levels are represented by straight horizontal lines instead of concentric circles. As in the atom diagram, the electron is represented as a small transparent circle. Light is represented as a wavy colored arrow. The change in energy level is shown with a dashed, straight gray arrow.

Four scenarios involving absorption of light and electron jumps are shown. This graph is almost identical to the diagram of the hydrogen atom on the left side of the infographic, but does not show the nucleus of the atom. The energy levels are represented by straight horizontal lines instead of concentric circles. As in the atom diagram, the electron is represented as a small circle. Light is represented as a wavy colored arrow. The change in energy level is shown with a dashed, straight gray arrow.

The energy that an electron needs in order to jump up to a certain level corresponds to the wavelength of light that it absorbs. Said in another way, electrons absorb only the photons that give them exactly the right energy they need to jump levels. (Remember when we said that photons only carry very specific amounts of energy, and that their energy corresponds to their wavelength?)

The three graphics on the right side are aligned to show the relationship between the electron drops, the color of light emitted, and the emission peaks on the graph.

A hydrogen atom is very simple. It consists of a single proton in the nucleus, and one electron orbiting the nucleus. When a hydrogen atom is just sitting around without much energy, its electron is at the lowest energy level. When the atom absorbs light, the electron jumps to a higher energy level (an “excited state”). It can jump one level or a few levels depending on how much energy it absorbs.

The spectrum is graphed as a line. The overall shape of the line resembles a bell curve cut off on the left and right sides. The curve begins on the far left with a brightness of about 0.83, increases to a peak of 1 at about 500 nanometers, and then decreases gradually to a low of about 0.7 on the right side of the graph.