We’ve shown how light reflects and obeys the laws of reflecting light when it strikes a surface. Light appears to travel in straight lines in a transparent medium.

When light passes through one transparent medium and into another, what happens?

Is it still travelling in a straight line, or has it altered its path? We’ll discuss some of our day-to-day experiences. 

Have you ever seen a pencil partially submerged in water in a glass tumbler? At the air-water interface, it appears to be displaced.

You may have noticed that when viewed from the sides, a lime kept in the water in a wine decanter appears to be larger than it actually is. How do you explain such occurrences?

You may have noticed that the bottom of a bucket, tank, or pond filled with water appears to be elevated. The letters appear raised when a thick glass slab is positioned over printed matter and viewed through the glass slab. What causes this to happen?

The first case is when the object is in the denser medium, and the observer is in the rarer medium.

  • When the light Ray travels from the object to the observer, it must pass through various mediums before reaching the observer. 
  • It moves away from the normal as it moves from a denser medium to a rarer medium, and the angle of refraction is greater than the angle of incidence. 
  • As a result, the object appears to be closer to the surface to the observer. This is something we can see in real life as well. 
  • When a coin is placed in a glass of water, the coil appears closer than it actually is.
  • When an object is in a rarer medium and the observer is in a denser medium, the object appears to be farther away from its original position. Thus the apparent height is greater than the real height.
  • When passing through a glass slab, there is a lateral shift.

Consider a light ray that passes through a glass slab with a thickness of t. In this case, there is a two-time change of the medium. The light Ray moves towards the normal when it enters the glass slab from air. When it moves away from the glass slab and into the air, it returns to incident direction.

Laws of refraction cause this phenomena. The law states that when a light ray moves from a rarer to a denser medium, it must move towards the normal and vice versa.

It’s possible that the emergent light Ray is parallel to the incident light Ray after two refractions. The lateral shift is the perpendicular distance between the incident and emerging light rays. 

How to test the ray optics with a transparent glass slab?

  • a) Secure a sheet of white paper to the drawing board with drawing pins in all four corners.
  • b) In the centre of the paper, place a glass slab and draw the EFGH boundary around it with a fine pencil.
  • c) Remove the glass slab. Any line AB that forms a 400 angle with the normal at point B, which is roughly the middle point of EF, should be drawn.
  • d) Move the glass slab to the right of the boundary line. Fix two pins P1 and P2 vertically on the line AB, at least 5cm apart, and one pin close to the slab.
  • e) From the opposite side GH, locate the image of these pins in the slab and position two pins P3 and P4 in line with the image of P1 and P2 as seen through the slab and at least 5cm apart.
  • f) To obtain the emergent ray, join the pricks of P3 and P4. Join BC to get the refracted ray by drawing a normal to GH at point C.
  • g) Determine the incidence angle and refraction angle. Produce AB forward and draw a perpendicular from C to P on the AB produced.
  • The lateral displacement is then equal to CP.
  • h) Repeat the experiment with different incidence angles of 500 and 600 

In conclusion:- 

The speed of light changes as it travels from one transparent medium to another transparent medium. If the speed of light slows as it enters the second medium, the second medium in which light has entered is said to have a higher optical density than the first. This change in optical density is also responsible for the bending of light when it enters a medium with a different refractive index.