Compact Disc (CD ) as Diffraction grating

Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit .

Italian scientist Francesco Maria Grimaldi coined the word " Diffraction " and was the first to record accurate observations of the phenomenon in 1660 .It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture.

Diffraction in CDs 

The development of compact disc involved a combination of laser Physics , Optics and Computer science . A CD consists of a series of alternating pits and and lands that spiral around the CD . When laser light is incident on a CD , the tracks of pits and lands cause the CD to behave as a diffraction grating . 

The pattern produced by the diffraction like phenomenon and the spacing of those pits and lands determines the amount of data on a CD .This distance between  adjacent tracks , commonly referred to as the track pitch .

Aim of the Experiment 

To 
1. demonstrate that CD has the characteristics of an optical grating .
2. find the grating constant or track pitch  of a CD
3. calculate the unknown wavelength of the laser .

Materials Required : 

Laser Pointer , Compact Disc , scissors , ruler ,protractor ,  pencil .

Theory : 

Diffraction (bending) of light is due to wave properties of light. It means that when a light wave encounters an obstacle, it does not propagate linearly behind the obstacle. During diffraction on an optical grating, formed by a system of a large number of parallel slits with equal width, a monochromatic light wave of a wavelength λ creates an interference pattern on a screen. The directions of interference amplification are determined by the angle θ, for which it applies

d (sinθ) = mλ

where d is the distance between two adjacent slits, called a grating constant or a grating period, and m = 0, 1, 2, … is the order of diffraction.

A recording on a CD is in the form of microscopic pits of different lengths that carry the information . These pits are placed in rows of the same width and equal distance , which form a diffraction grating on the mirror surface of the CD .

OBSERVATIONS:

Part - I : To calculate the track pitch or diffraction grating of a CD 

S. No	D ( cm )	Distance between two corresponding maximas  X ( cm )	Tanθ =  x /D	θ ( in degrees )	Sin θ	d= λ/ Sin θ ( μm )
1	37.4	13.9	0.371	20.35	0.347	1.53
2	31.5	12.6	0.400	21.80	0.371	1.43
3	32	11.8	0.368	20.20	0.345	1.54
4	31	10.8	0.348	19.18	0.328	1.62
5.	33	12.5	0.378	20.70	0.353	1.50

Results:  Mean of the grating constant= (1.53 +1.43 + 1.54 +1.62 + 1.50 ) /5

= 1.53 μm

Track Pitch ( Experimentally ) = 1.53 μm

Track Pitch ( Actual ) = 1.6 μm

Error = 0.07 μm

Part - II: Finding unknown wavelength

S. No	D ( cm )	Distance between two consecutive maximas (cm)	Tanθ= x/D	θ ( in degrees)	Sin θ	λ= d Sin θ (nm)
1.	100	48.15	0.48	25.64	0.43	657.9

Unknown wavelength : 657.9 nm

Precautions: 

Follow the safety rules while working with the laser beam .

 Experimental Set- Up :      

Diffraction Pattern using CD

Credits : My Students 

1. Rucha Kasture

2. Archit Malhotra

3. Vidushi Saxena