Page 112 - Physics - XII

Result

1. The ray of light changes its path emerging out of glass slab which proves that refraction happens when
light travels from one medium to another. The emerging ray is parallel to the incident ray, i.e. ∠ = ∠e.
2. The lateral deviation of the emergent ray with respect to the incident ray is directly proportional to
the thickness of the glass slab.
3. The lateral displacement increases with the angle of incidence (i).

Precautions
1. The pins should be fi xed vertically on the drawing board.
2. The glass slab should be clean and of uniform thickness.

3. Use sharp pencils for making lines.
4. The glass slab should be kept lengthwise at the centre of the drawing sheet.
5. Measurement of angles should be done accurately.

6. For better accuracy, distance between the pins should be 8 cm or more while locating the angle of
incidence and angle of emergence.
7. While viewing the pins, one eye should always be kept closed.

Sources of Error

1. Measurement of angles may not be accurate. 2. Glass slab may not have uniform thickness.

ACTIVITY - 5
Aim
To observe diff raction of light due to a thin slit.

Apparatus and Materials Required

Two razor blades with sharp edges and a sodium lamp

Principle/Theory
The phenomenon of bending of light around the sharp corners of an obstacle and spreading into the regions
of the geometrical shadow is called diff raction.
When a narrow slit of small width is illuminated with monochromatic light and a screen is placed
at a suitable distance, diff raction of light due to the slit is obtained on the screen. The diff raction
pattern consists of a central maxima surrounded on either side by a number of alternate dark and bright
fringes called secondary maxima and minima.

Procedure
1. Hold two razor blades side by side keeping their sharp edges

close and parallel to each other as shown in Fig. 5.1 to form a
narrow slit in between.
2. Switch on the sodium lamp and keep the distance of the sodium
lamp at a distance of about 2 m from the blades.
3. Observe the glowing of sodium lamp through the slit formed Fig. 5.1: To observe the diff raction of
between the two sharp edges of the blades. light through thin slit using
two razor blades.
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