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Human Eye and Colourful World

The human eye

 When a ray of light is incident on a rectangular glass slab, after refracting through the slab, it gets displaced laterally. As a result, the emergent ray comes out parallel to the incident ray. Does the same happen if a ray of light passes through a glass prism?

Unlike a rectangular slab, the sides of a glass prism are inclined at an angle called the angle of prism. Therefore, a ray of light incident on its surface, after refraction, will not emerge parallel to the incident light ray (as seen in the case of a rectangular slab).

Refraction of light through a glass prism

To observe the refraction of light through a glass prism, we can perform the following activity.

Take a triangular glass prism, paper sheet, and a few drawing pins. Fix the sheet on a drawing board with the help of drawing pins. Now, place the glass prism on the sheet and draw the outline MNP of the prism on the sheet (as shown in the figure). Draw a straight line AB on the sheet in such a way that it makes some angle with the face MN of the prism. Now, fix two pins on this line and mark them as R and S respectively.

Now, observe the pins R and S through the other side of the prism. Move your head laterally to see the two pins R and S in a straight line. Fix a pin on the sheet near the prism on your side and mark it as T.

Repeat the same step and try to observe the three pins R, S, and T in a straight line. Fix another pin on the sheet so that all four pins appear to be in a straight line when looked through the prism. Draw a straight line CD that passes through the third and the fourth pin i.e., T and W respectively (see figure).

Now, remove the prism and join points B and C. The straight line AB, BC, and CDshows the path of the light ray. It is clear that the path of light is not a straight line since light bends towards the base NP.

What causes the light to bend when passed through a prism?

Light bends because of refraction that takes place at points B and C respectively, when it tries to enter and emerge from the prism.

Now, draw a straight line normal to side MN and let it pass through point B. Similarly, draw a straight linenormal to side MP and let it pass through point C.

Here, line AB = Incident ray

Line BC = Refracted ray

Line CD = Emergent ray

Angle i = Angle of incidence

Angle r = Angle of refraction

Angle e = Angle of emergence

Angle= Angle of deviation

Hence, you will get the path of light ray AB when it travels through a glass prism. The ray AB will bend towards the normal HI at point B and follow the path BC. Again, it bends away from the normal GI at C, when it tries to emerge from the prism. This is because the refractive index of air is less than that of glass. Thus, the incident ray AB will not follow a straight line BE.

The extent of deviation o...

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