Refraction through a rectangular glass slab काँच के आयताकार स्लैब से अपवर्तन
Aim: To trace the path of a ray of light through a rectangular glass slab for different angles of incidence, measure the angle of refraction, the angle of emergence and the lateral displacement, and verify that the emergent ray is parallel to the incident ray.
Apparatus उपकरण
- Rectangular glass slab
- Drawing board and white sheet
- Drawing pins and four or more steel pins
- Protractor (least count 1°)
- Half-metre scale (least count 0.1 cm) and a sharp pencil
Theory सिद्धांत
When light passes from air into glass it bends towards the normal; when it comes out into air again it bends away from the normal. Snell's law: sin i / sin r = n, the refractive index of glass (about 1.5). Because the two faces of the slab are parallel, the bending at the second face exactly undoes the first, so the angle of emergence e equals the angle of incidence i and the emergent ray is parallel to the incident ray. It is only shifted sideways by the lateral displacement d = t sin(i − r) / cos r, where t is the thickness of the slab.
Procedure विधि
- Fix a white sheet on the drawing board and place the slab in the middle. Draw its outline ABCD.
- Draw a normal at a point E on face AB and draw the incident ray at the chosen angle of incidence i.
- Fix two pins P and Q on the incident ray, a few centimetres apart.
- Look through face CD and fix pins R and S so that they appear in one straight line with the images of P and Q.
- Remove the slab and pins. Join R and S to meet CD at F: this is the emergent ray. Join E and F: this is the refracted ray.
- Draw the normal at F and measure ∠r and ∠e with the protractor. Extend the incident ray and measure the perpendicular distance between it and the emergent ray (lateral displacement).
- Repeat for three or more angles of incidence between 30° and 60°.
Simulated experiment अनुकरण प्रयोग
Your slab is 6 cm thick. Choose the angle of incidence, then read r and e on the protractors and d on the scale (1 small division = 1 mm).
Observation table प्रेक्षण तालिका
Record three or more angles between 30° and 60°. Angles are checked to ±1°, d to ±1.5 mm.
| S. No. | ∠i | ∠r | ∠e | d (cm) | sin i / sin r | Remove |
|---|---|---|---|---|---|---|
| Record an angle of incidence above. | ||||||
Enter the angle of refraction to calculate the refractive index.
Precautions सावधानियाँ
- Use a slab with smooth, parallel faces and no bubbles or scratches.
- Keep the pins vertical and at least 5 cm apart.
- Look at the feet of the pins, not their heads, when lining them up.
- Keep the angle of incidence between 30° and 60°.
- Draw thin lines with a sharp pencil and keep the slab exactly on its outline.
Sources of error त्रुटि के स्रोत
- Pins may not be exactly vertical, so their images do not line up perfectly.
- The slab may shift on the sheet while the pins are fixed.
- The protractor can only be read to 1°, which matters most at small angles.
- Thick pencil lines make the measurement of lateral displacement less accurate.
Viva voce questions मौखिक प्रश्न
Q1. Why does a ray of light bend when it enters glass from air?
Light travels more slowly in glass than in air. The change of speed at the boundary makes the ray bend towards the normal.
Q2. How are the angle of incidence and the angle of emergence related for a glass slab?
They are equal (i = e), because the opposite faces of the slab are parallel.
Q3. What is lateral displacement and on what does it depend?
The perpendicular distance between the incident ray (extended) and the emergent ray. It increases with the thickness of the slab, the angle of incidence and the refractive index.
Q4. State the laws of refraction.
The incident ray, the refracted ray and the normal lie in the same plane, and sin i / sin r is constant for a given pair of media (Snell’s law).
Q5. What happens if light falls on the slab along the normal (i = 0°)?
It passes straight through without bending, so there is no lateral displacement.