When we say that the index of refraction of glass is 1.5, we mean the refractive index for glass with respect to air when illuminated by a white light. It is the ratio between the speed of light in air to that in a glass. Consequently, the index of refraction of glass with respect to water will vary, as shown above. Also, the refractive index of glass for red and violet light differs due to the difference in wavelength of the two colours.

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Moreover, the refractive index anticipates how a light ray will change its direction at the junction of separation of two different media. We can observe this phenomenon when a light ray from a smaller index of refraction reaches a medium with a greater index; it bends toward the normal. In the opposite case, i.e., the light ray tends to move away from the normal when it travels to a medium with a smaller refractive index.

Optical Engineering is a monthly peer-reviewed scientific journal covering research, development, and uses of optical science and optical engineering, published by SPIE. The editor-in-chief is Adam Wax (Duke University, USA). Past editors include Michael Eismann.[1]

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When light strikes a surface and returns to the same media, it is said to be reflected. The phenomenon is called reflection. However, when light strikes a surface, and a part of it gets reflected while the other part travels into the other media, it is said to be refracted. The phenomenon is called refraction. So, what do you understand by refractive index? The topic is important for Class 10, NEET, and JEE aspirants as it is often a part of the NEET Syllabus and JEE Main Physics Syllabus. Learn about the refractive index of glass, how it varies, and the key factors that affect the refractive index from an expert’s point of view.

Since PQ and RS are parallel to one another and refraction is occurring at parallel surfaces, we can apply the reversibility principle. Thus, the incident angle equals the emergent angle. Also, the incident ray runs parallel to the emergent ray. However, they are not along the same line. We can notice that the emergent ray is laterally displaced.

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However, the refractive index can be less than one for a particular medium if a light ray passing through it has a phase velocity faster than the speed of light.

Refractive index is directly proportional to the optical density of the material. Since flint glass is denser than crown glass, the refractive index of flint glass will be higher than that of ordinary crown glass. The refractive index of crown glass is approximately 1.5, while the refractive index of flint glass is 1.63 approximately.

The refractive index of glass is inversely proportional to the light rays’ wavelength. Since the wavelength increases on moving from violet to red in VIBGYOR, the colour red has a maximum wavelength while the colour violet has a minimum wavelength. Therefore the refractive index of glass is maximum for violet colour.

1. Medium’s thickness: The lateral displacement is more for a thicker medium (e.g. a thicker glass block). 2. The incident angle: The lateral displacement is more for a greater angle of incidence.  3. Refractive index: The lateral displacement is more for a higher index of refraction.   4. The light’s wavelength: Since the wavelength is inversely proportional to the index of refraction, lateral displacement is also inversely proportional to the light’s wavelength. Lateral displacement is more for a light with less wavelength.

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Step 7: Next, you will have to remove the pins and mark their points on the sheet of paper using a fine pencil. Withdraw the glass block and join the marked points c and d with a line BC that meets the RS side of the rectangle at a point B.

Step 5: Place the block again on the same boundary lines. Fix two pins’ a’ and ‘b’ on AO’s incident line. The pins should be approximately 5 cm apart.

The statement that the refractive index of glass is 1.5 implies that when a light ray travels in glass, its speed is 1.5 times slower than the speed of light in a vacuum. However, we assume light to be a white light that illuminates the glass surface as the refractive index would differ for red or violet coloured light rays.

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The sine ratio of different values of angle of incidence and refraction will come out to be a constant. This constant value represents the refractive index of the glass.

The ratio of the speed of light in a vacuum or air to the speed of light in a given medium. Thus, the refractive index tells us how fast a light ray can travel in a medium.

Step 6: Now, look from the other side RS of the glass block, keeping your eye as close to the plane as possible. Fix two more pins, ‘c’ and ‘d’, such that the four pins a, b, c, and d appear in the same line when seen through the block.

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So, the higher the index of refraction of a medium, the higher is the optical density, and the slower will be the speed of light.

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Step 2: Now remove the glass block, and on the boundary line PQ, mark as point O nearly towards the centre of the line PQ.

Step 9: Now, measure the angle of incidence and the angle of refraction. Now read the value of sin i and sin r and calculate the ratio of sin i/ sin r. If you do not want to measure the angles, there is an alternative method.

The ratio between the sine of the incident angle to the sine of the refracted angle is constant for a given pair of media, i.e.,

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Step 10: Repeat the same procedure for different angles of incidences, such as angles 45° and 60° and record the values of i and r in a table. Create columns for i, r, sin i, sin r, and sin i/ sin r or DF /EG.

The refractive index value is usually greater than or equal to 1, as the speed of light in vacuum/ air is always greater than the speed of light in any other medium.