Page 113 - Physics - XII

4. The dark and bright fringes will be observed. In case an ordinary electric lamp (in place of sodium
lamp) is used, coloured fringes will be observed.

5. Now, decrease the width of the slit between the sharp edges of the two blades. When the width of the
slit decreased, the fringes become wider. In practice, as the two edges of blades are often not parallel to
each other when we hold them with the two hands, the thickness of diff erent parts of the slit is diff erent
and hence, the fringes of diff erent widths will be observed.

Result

Light waves incident on very small apertures bend around the corners which indicates the phenomenon
of diff raction.

Precautions

1. The edges of two blades should be held parallel and close to each other.
2. The source of light should be kept at least at a distance of 2 m from the thin slit between the two blades.

ACTIVITY - 6

Aim
To study the nature and size of the image formed by a concave mirror, on a screen by using a candle and a
screen (for diff erent distances of the candle from the mirror).

Apparatus and Materials Required
A concave mirror with holder, an optical bench with three uprights, a metre scale, a candle, a cardboard
screen, and a matchbox

From
infi nity

C F P O C I F P

(a) Object at infi nity (b) Object beyond C

O

C F P I C O F P

(c) Object at C (d) Object between C and F

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