Through this glass slab experiment, we can easily identify refraction of light. Through this experiment we can find the lateral displacement of light ray after refraction.
What is refraction?
The phenomenon of change in path of light as it moves from on transparent medium to another transparent medium, is called refraction of light.
Define lateral displacement of light
The perpendicular distance between incident ray and emergent ray is called lateral displacement.
Object
To show the refraction of light when light moves from air to glass and from glass to air.
Material
Glass slab, drawing board, Alpins .
Method
We take a transparent glass slab PQRS and place it on a white paper sheet and mark a boundary PRRS with pencil
Now we remove the glass slab and draw a line AB, an incident ray at angle of 40° on the side PQ and draw a normal at the point B (a perpendicular line on side PQ)
After that put the glass slab again on the boundary line and fix two pins 1 and 2 on the line AB at distance of 5 cm.
Now we can see image to both pins from the opposite side RS and fix two pins 3 and 4 in such a way that they are in the line with image of 1 and 2.
Now draw a line CD on the side RS joining pins 3 and 4, it is emergent ray and also draw a normal on the point C in the side RS.
Now join point B and C and measure angle of incidence, angle of refraction and angle of emergence.
The lateral displacement is obtained by extending the ray AB in a dotted line which is parallel to ray CD.
To measure the lateral displacement, draw a perpendicular line from point C on dotted line (extended line AB) to meet at point P
So the lateral displacement = CP
Now we repeat the experiment for angles 45° and 60°.
| S.No. | Angle of incidence | Angle of refraction | Angle of emergence | ∠i-∠e |
| 1 | 40° | 38° | 40° | 0 |
| 2 | 45° | 44.8° | 45° | 0.2 |
| 3 | 60° | 59.8° | 60° | 0.2 |
Conclusion
When a ray of light moves from an optically rarer medium to optically denser medium, it bends towards the normal and when it moves from optically denser medium to optically rarer medium, it bend away from the normal.
The ratio of sine of angle of incidence to the sine of angle of refraction is constant for the light of a given colour and given pair of media.
`\frac{sin i}{sin r} =constant`
Viva questions
1.What is the object of the glass slab experiment?
Ans – The glass slab experiment show the refraction of light when it passes form one transparent medium to another transparent medium.
2.What do you mean by optically rarer medium?
Ans – An optically rarer medim is a transparent medium through which light travels faster than in another medium and it has lower refractive index.
3.What is an optically denser medium?
Ans – An optically denser medium is a transparent medium in which light travels slowly than in another medium and it has higher refractive index.
4.What happens when a light ray travel forms rarer medium to denser medium?
Ans – When a light ray travel from a rarer medium to a denser medium, it bends towards the normal.
5.What is the angle of refraction when angle of incidence is zero?
Ans – When the angle of incidence is zero, the angle of refraction is also zero.
6.What is optical density?
Ans – Optical density also known as refractive index that show how a medium refract or bends a light ray.
7.Can angle of refraction be greater than the angle of incidence?
Ans- No angle of refraction cannot be greater than the angle of incidence.
8.Which factors affect the lateral displacement produced by a glass slab?
Ans- lateral displacement produced by glass slab depends on thickness of glass slab, refractive index of glass, angle of incidence and wavelength of light.