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.
|Table of Contents|
|What is the Refractive Index?|
|Refractive Index Formula|
|What Do You Understand by the Refractive Index of a Glass?|
|Experimental Determination of Refractive Index of Glass|
|Factors Affecting the Refractive Index of Glass|
What is the Refractive Index?
According to Snell’s Law,
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.,
sin isin r= 12
The constant from the above ratio is called the refractive index. It is dimensionless and is represented by mew ().
We can also define the index of refraction in terms of the speed of light,
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.
So, speed of light in vacuum or air (c)speed of light in a medium=
speed of light in medium 1speed of light in medium 2= 1 2
Refractive Index Formula
If we represent the refractive index by the symbol n, the formula of the refractive index is as follows:
- n is the refractive index
- v is the light’s velocity in a medium
- c is the light’s velocity in a vacuum/air ( 3 × 108 m/s)
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.
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.
What Do You Understand by the Refractive Index of a Glass?
Considering the general definition, we can define the refractive index of glass as follows:
|Definition: The refractive index of glass describes a ratio between the speed of light in a vacuum or air and the speed of light in a glass medium.|
speed of light in air speed of light in glass= glass
We know that the speed with which light travels in the air is 3 x 108 m/s.
While the speed differs in glass, i.e., light travels at the rate of 2 x 10 8 metres per second in a glass.
So, we can calculate the refractive index of glass with respect to air as follows
= 3 108 2 108= glass
3/2 =1.5 = glass
Thus, we can conclude that the glass refraction index is 3/2 or 1.5.
Refractive Index of Glass with Respect to Water
We can also determine the refractive index of glass when placed in water, using the general refractive index formula
speed of light in medium 1speed of light in medium 2= 1 2
In the first medium, i.e., water, the speed of light = 2.2×108m/s
While in the second medium, i.e., glass, the speed of light = 2×108m/s
Refractive index = speed of light in waterspeed of light in glass= 1 2
= 2.2×108 m/s /2×108 m/s
|Did you know?
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.
The refractive index of vacuum/ air at STP is 1.
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.
Experimental Determination of Refractive Index of Glass
The steps for determining the refractive index via the refractive index of glass slab experiment are as follows:
Step 1: Place a glass block on white paper and draw its boundary. Marks the four points as PQRS.
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 3: Draw a normal NOM at point O, passing through the line PQ.
Step 4: Now, draw a line AO incident at the point O at an angle ‘i’ with the normal NOM. (Let us take i = 30 degrees)
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.
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.
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 8: Join the points O and B with a straight line. OB gives the path of the ray inside the glass block.
From the above steps, we have the following
Normal = NOM
Incident ray = AO
Refracted ray = OB
The angle of incidence (i)= Angle AON
The angle of refraction (r) = Angle BOM
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.
- Draw a circle with a suitable radius taking point O as the circle’s centre. The circle intersects the incident ray at point D and the refracted at point E.
- Draw normal DF and EG from points D and E, respectively, on NOM.
- Measure the length of Df and EG and find DF/ EG. The ratio will be constant, giving us the refractive index of glass.
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.
Conclusions from the refractive index of glass slab experiment:
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.
Lateral Displacement of Light Ray
In the above experiment, if we draw a normal N’BM’ to the surface RS at point B, we can say that at the surface RS,
- OB is the incident ray
- OB is the refracted ray
- BC is the emergent ray
- Angle CBM’ is the angle of emergence
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.
Thus, lateral displacement refers to the perpendicular distance between the two parallel rays: incident and emergent.
The formula of lateral displacement is as follows:
t sin (i-r)cos r= Lateral Displacement
Factors Affecting the Refractive Index of Glass
- Optical Density: The refractive index of glass varies for different optical densities. When illuminated by white light, the index of refraction of ordinary crown glass is 1.5, while it is 1.63 for medium flint glass and 1.49 for acrylic glass.
- Temperature: The refractive index of glass, i.e., 1.5, is calculated at the standard temperature. However, the refractive index will decrease when we increase the temperature because of an increase in the speed of light.
- The wavelength of light: The speed of light of all colours is the same in air/vacuum. However, in glass or any other medium, the speed of light varies with a colour change. The speed of red light is maximum while that of violet light is minimum. We know that the red light’s wavelength is more than that of violet, so the refractive index of the glass will be lesser for a higher wavelength. Thus, the refractive index of glass for red light will be lesser than that of violet light.
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.
1. Which one has a higher refractive index: crown glass or flint glass?
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.
2. What does the refractive index of glass 1.5 mean?
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.
3. What are the factors on which lateral displacement depends?
Lateral displacement varies depending on the following factors:
- Medium’s thickness: The lateral displacement is more for a thicker medium (e.g. a thicker glass block).
- The incident angle: The lateral displacement is more for a greater angle of incidence.
- Refractive index: The lateral displacement is more for a higher index of refraction.
- 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.
4. For which colour the refractive index of glass is maximum?
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.