REFRACTION THROUGH GLASS SLAB (lateral and vertical shifts)

 REFRACTION THROUGH GLASS SLAB:

A thin glass slab is formed when a medium is isolated from its surroundings by two plane surfaces parallel to each other. Let us determine position and nature of the image formed when the slab is placed in front of an object.

 

Lateral shift:

 

Understanding with an Activity:

 Material required: plank, chart paper, clamps, scale, pencil, thin glass slab and pins.

 Procedure:

Place a piece of chart (paper) on a plank. Clamp it. Place a glass slab in

the middle of the paper. Draw border line along the edges of the slab by

using a pencil. Remove it. You will get a figure of a rectangle. Name the

vertices of the rectangle as A, B, C and D.

Draw a perpendicular at a point on the longer side (AB) of the rectangle.

Again keep the slab on paper such that it coincides with the sides of the

rectangle ABCD. Take two pins. Stick them on the perpendicular line to

AB. Take two more pins and stick them on the other side of the slab in

such a way that all pins come to view along a straight line. Remove the slab

from its place. Take out the pins. Draw a straight line by using the dots

formed by the pins such that it reaches first edge (AB) of the rectangle.

You will get a long straight line.

The light ray that falls perpendicular to one side of the slab surface

comes out without any deviation.

Now take another piece of white chart on the plank. Clamp it. Place a glass slab in the middle of the paper. Again draw a border line along the edges of the slab by using a pencil. Remove the slab and name the vertices of the rectangle formed as A,B,C and D. Draw a perpendicular at a point on the longer side AB. Now draw a line, from the point of intersection where side AB of rectangle and perpendicular meet, in such a way that it makes 30⁰ angle with normal. This line represents the incident ray falling on the slab and the angle it makes with normal represents the angle of incidence.

Now place the slab on the paper in such way that it fits in the rectangle drawn. Fix two identical pins on the line making 30⁰ angle with normal such that they stand vertically with equal height. By looking at the two pins from the other side of the slab, fix two pins in such a way that all pins appear to be along a straight line. Remove slab and take out pins. Draw a straight line by joining the dots formed by the pins up to the edge CD of the rectangle. This line represents emergent ray of the light.

Draw a perpendicular ON to the line CD where our last line drawn meets the line CD. Measure the angle between emergent ray and normal. This is called angle of emergence.

Incident ray and emergent ray will be parallel to each other, Therefore angle between them is zero. And the distance between these two parallel rays is called ‘lateral shift’

 

  Lateral shift depends upon  

1) The thickness of the glass slab: The thicker the glass slab, More is the lateral displacement.

2) Angle of incidence: The lateral shift increases with increase in angle of incidence.

Angle of incidence=Angle of emergence

 Vertical shift:

 Understanding with an activity:

Measure the thickness of the slab. Note it in your notebook. Take a white chart and fix it on the table. Take the slab and place it in the middle of the chart. Draw its boundary. Remove the slab from its place. The lines form a rectangle. Name the vertices as A,B,C and D. Draw a perpendicular to the longer line AB of the rectangle at any point on it. Place slab again in the rectangle ABCD. Take a pin. Place at a point P in such a way that its length is parallel to the AB on the perpendicular line at a distance of 15cm from the slab. Now take another pin and by looking at the first pin from the other side of the slab try to place the pin so that it forms a straight line with the first pin. Remove the slab and observe the positions of the pins.

 

Draw a perpendicular line from the second pin to the line on which first pin is placed. Call the intersection point Q. Find the distance between P and Q. We may call it vertical shift.

To find this, do the same activity for another distance of the pin from the slab. You will get the same vertical shift. We could use a formula to find out refractive index of the glass.

 

µ= Thickness of the slab / (thickness of slab – vertical shift)