# Demos on diffraction

## Light through a rectangular hole

For large size holes, the light propagation is governed by geometrical optics (rectilinear propagation). However, you can create situations in which, at the first glance, some surprises occur. This experiment is one such example.

You need a card board fixed vertically with an arrangement to open a rectangular hole, an electric bulb holder, a filament bulb and a CFL, a vertical white screen.

1. Arrange the bulb holder, the cardboard and the white screen in line as shown in the figure. Keep the distance between the bulb holder and the cardboard about 30 cm and, between the cardboard and the screen also about the same. Make rectangular hole thinnest. To do this keep the given hinges closed.
2. Put a CFL bulb in the bulb holder and look at the screen. Do you see the image of the CFL? Is the image inverted or erect?
3. Now put a filament bulb in place of the CFL. What do you see on the screen? Do you see the image of the rectangular hole? Explain why the two behave differently.
4. Now open the hinges to widen the rectangular hole. Repeat step-3. what changes do you see.

## Light through a small hole

If you look at the sunlight coming through the leaves of a tree and falling on the ground, you find similar looking circular or elliptical spots. The geometry of the spaces between the leaves does not show up in the pattern. This experiment creates a similar situation in the classroom and stimulates discussion.

A bulb holder with aperture limiting slit attached, a filament bulb, a movable white screen, cardboards with holes of different shapes, such as triangle, rhombus, plus sign etc.

1. Place the bulb in the holder and place it on a table. Place the screen in front of the bulb at a distance about of 20 cm. The length of the filament should be perpendicular to the line joining the bulb and the screen.
2. Now bring a cardboard with hole between the bulb and the screen so that light only passes through the hole. Keep the cardboard close to the screen, say at 4-5 cm. See the shape of the light spot on the screen. Is it same as the shape of hole? Try with different cardboards and see that each time the spot has the same shape as the hole.
3. Keeping the cardboard at the same place, displace the screen away from the cardboard. Take it up to 60 cm. How does the shape of the spot change?
4. Use different cardboards and compare the shapes when its distance from the screen is large. Write your conclusions.

## Diffraction from a CD/DVD

You must have seen the bright colours on a CD/DVD surface when viewed in bright light. However, much more physics can be learnt from diffraction of a monochromatic light beam from CD/DVD surface. A CD has a large number of concentric tracks.

You need a CD, a DVD, a laser source, and a white screen.

1. Put the CD on a horizontal surface and fix the laser torch to send the light beam obliquely on the surface. See the pattern on the screen.
• Question: What kind of pattern do you observe if the plane of incidence is parallel to the circular tracks and perpendicular to the track?
2. You will see separate bright spots when diffraction pattern is formed. Observe the separation between consecutive bright spots. This is inversely proportional to the separation between successive pits along a circular track on the CD.
3. Now send the beam on inner circles. Look at the separation between the bright spots on the screen. Shift the CD to gradually go to outer circles.
• Question: As you go towards outer circles, does the fringe separation increase or decrease?
• Question: What do you conclude about the number of pits per millimeter for inner and outer tracks. (you can makes calculation to check whether the number of tracks are equal on all tracks)
4. Now use a DVD in place of CD and do the same.
• Question: For the tracks at the same radial distance, does DVD diffraction gives narrower fringes or wider fringes as compared to the CD diffraction pattern?
• Question: Is the separation between consecutive pits on DVD larger or smaller as compared to that on the CD?

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