Lateral Deviation of a Light Beam by Glass Slab

Lateral Deviation of a Light Beam

Aim

To observe refraction and lateral deviation of a beam of light incident obliquely on a glass slab.

Apparatus and Material Required

• Drawing board,
• rectangular glass slab,
• white sheet of paper,
• adhesive tape (cello-tape) or drawing pins,
• a metre scale,
• alpins,
• protractor,
• sharp pencil and
• eraser.

Principle

A ray of light incident on a rectangular glass slab bends at the interface of two optical media, here, air and the glass. The emerging ray out of the slab is parallel to the direction of the incident ray. This ray suffers only a lateral displacement. The lateral deviation is proportional to the thickness of the glass slab for a given angle of incidence and a pair of media.

Procedure

1. Fix a white sheet of paper on the drawing board with the help of cello-tape or drawing pins.
2. Place the glass slab at the centre of the paper sheet and mark its boundary ABCD.
3. Draw a normal at a point F on the face AB. Draw a line EF inclined at angle \(i\) with the normal. EF and \(i\) represents incident ray and angle of refraction respectively.
4. Fix two alpins \(P\) and \(Q\), about 8 to 10 cm apart, vertically on the line EF.
5. Observe the images of the two pins through the face opposite of the glass slab. Fix two more alpins R and S about 8 to 10 cm apart vertically on the sheet of paper in such a way that tips of alpins P, Q, R and S are collinear.
6. Remove the glass slab and circle the pin prick positions of the alpins. Join R and S by a straight line and extend it to touch the slab boundary CD at point G.
7. Join points F and G. Straight line FG represents refracted ray.
8. Draw a normal to the side CD at the point G. Measure the angle of emergence, \(e\).
9. Extend the line EF (incident ray) by dotted line to intersect the face CD at O of the glass slab.
10. Draw a perpendicular OL to the line RS meeting the line at L.
11. Verify whether the emergent ray GH is parallel to the incident ray EF.
12. OL is lateral deviation \(d\). Record its value. Also note down the thickness of the glass slab.
13. Repeat the steps 3 to 12 for various angles of incidence.
14. Record observations in tabular form.

Observations

Least count of the protractor = 1°
Least count of the metre scale = 1 mm = 0.1 cm

Result

The ray of light emerging from a glass slab is parallel to the incident ray. However, it is laterally deviated.

Precautions

• Fix alpins vertically to the plane of paper.
• Distance PQ and RS should be about 10 cm in order to locate incident and emergent rays with greater accuracy.
• Same angle of glass slab should be used for all observations.
• Position of the glass slab should not be disturbed for a given set of observations.

Sources of errors

• The pins on emergent ray are not collinear with those on incident rays.
• Dislocation of the glass slab while taking reading.

Conclusion

The experiment demonstrated refraction and lateral deviation of light through a glass slab. The emergent ray is parallel to the incident ray. The angle of refraction increases with angle of incidence. The lateral deviation increases with the angle of incidence.

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Reference: NCERT Lab Manual

Lateral Deviation of a Light Beam