Chapter 5 - Optics

Question 1

What is refraction?

Answer 1

Change in direction that occurs when light passes at an angle across a boundary between two transparent substances. Due to change in speeds

Question 2

What is the normal?

Answer 2

Line perpendicular to a boundary between two materials or a surface

Question 3

What happens to a ray of light at the boundary of a more dense substance?

Answer 3

Light ray slows down and therefore bends towards the normal.

Question 4

What happens to a ray of light at the boundary of a less dense substance?

Answer 4

Light ray speeds up and therefore bends away from normal.

Question 5

What is approximately the refractive index of air?

Answer 5

1

Question 6

What is the equation for refractive index involving speeds of light?

Answer 6

n = speed of light in a vacuum/ speed of light in the transparent substance

Question 7

What is snells law for refraction at a boundary?

Answer 7

n2/n1 = sini/sinr

Question 8

A light ray crosses the boundary between water of refractive index 1.33 and a glass of refractive index 1.50 at an angle of incidence of 40 degrees. Calculate the angle of refraction of this light ray.

Answer 8

n1 = 1.33 , n2 = 1.50 , sini = 40 degrees
sinr = 1.33 x sin40 / 1.5
= 0.57
r = 35 degrees

Question 9

Do smaller wavelengths refract less or more?

Answer 9

More
Dispersive effect occurs because of the speed of light in glass depends on the wavelength. Violet light travels more slowly than red light, so has a greater refractive index.

Question 10

What is the critical angle?

Answer 10

Angle where light ray refracts along the boundary. If angle of incidence exceeds the critical angle total internal reflection occurs.

Question 11

What are the conditions for total internal refraction to occur?

Answer 11

1. ) The incident substance has a larger refractive index than the other substance
2. ) Angle of incidence exceeds the critical angle

Question 12

What does an optical fibre do? How does this work?

Answer 12

Allows pulses of light that enter at one end, from a transmitter, to reach the receiver on the other end.
Light rays are internally refracted each time it reaches a boundary - as the angle of incidence exceeds the critical angle.

Question 13

What features of optical fibres allow it to carry out it’s function?

Answer 13

• Core surrounded by layer of cladding of lower refractive index to allow total internal refraction to occur and to reduce light loss from the core (that would reduce amplitude). Also prevents light from passing to other fibres in direct contact.
• Highly transparent to minimise absorption of light, which would reduce amplitude of the pulses.
• Core must be narrow to prevent modal dispersion
• Monochromatic light used to prevent material dispersion

Question 14

What is modal dispersion? What is done to prevent this

Answer 14

Occurs in wide core fibres because light travelling axis of the core travels a shorter distance per metre of fibre than the light that repeatedly undergoes total internal reflection
Core is made very narrow to prevent this

Question 15

What is material dispersion?

Answer 15

Speed of light in glass of optical fibre depends on wavelength of the light. Differences in speed would cause white pulses in optical fibres to become larger, as e.g. violet falls behind red.
Monochromatic light used to prevent this.

Question 16

What experiment is carried out to prove interference of light?

Answer 16

Youngs double slit experiment

Question 17

Briefly describe young’s double slit experiment

Answer 17

Illuminate a narrow single slit using light source which illuminates two closely spaced parallel slits (double splits) using light source. The two slits act as coherent sources of wave (emit waves of constant phase difference and the same frequency). Alternate white and dark fringes (young’s fringes) can be seen on a white screen placed where the diffracted light from the double slit overlaps.

Question 18

Why are the fringes formed in young’s double slit experiment?

Answer 18

Due to interference of light from the double slits

Question 19

Why are bright fringes formed?

Answer 19

Light from one one slit constructively interferes with the light from the other slit (arrive in phase)

Question 20

Why are dark fringes formed?

Answer 20

Light from one slit destructively interferes with the light from the other slit (arrive 180 degrees out of phase/ anti-phase)

Question 21

What is the equation for fringe spacing?

Answer 21

fringe spacing = wavelength x distance from the screen to the slits/ split spacing

Question 22

Give the equation for path difference in this situation

Answer 22

S(1)P - S(2)P = m x wavelength

Question 23

What would the equation be if the light emitted from S(1) and S(2) arrives out of phase at P?

Answer 23

S(1)P - S(2)P = (m+1/2) x wavelength

Question 24

Why are lasers dangerous?

Answer 24

Lens of eye can focus the laser to a very fine spot (aided by it’s monochromatic property) on the retina and an intense concentration of light would destroy the retina.

Question 25

What does coherent mean?

Answer 25

Waves of constant phase difference and same frequency

Question 26

What is diffraction?

Answer 26

Spreading out of waves when they pass through a gap or by an edge

Question 27

Briefly describe diffraction of light by a single slit

Answer 27

Monochromatic/white light shone at a slit. The diffracted light forms a pattern that can be observed on a white screen. Shows a central fringe (x2 the size) with further fringes either side of the central fringe that are less intense (all the same size)

Question 28

What is the equation for width of central fringe in single slit diffraction?

Answer 28

w = wavelength of light/ width of the single slit x (2 x distance from the slit to screen)

Question 29

What is the effect of using greater wavelength light on the diffraction pattern?

Answer 29

Fringes get wider

Question 30

What is the effect of making the slit narrower on the diffraction pattern?

Answer 30

Makes fringes wider

Question 31

What makes waves diffract more?

Answer 31

• Gap is made narrower
• wavelength is made larger

Question 32

What is a diffraction grating?

Answer 32

Plate with many closely spaced parallel slits on it

Question 33

When a parallel beam of monochromatic light is directed normally at a diffraction grating, why is light only transmitted by the grating in certain directions?

Answer 33

Because:

• Light passing through each slit is diffracted
• Diffracted light waves from adjacent slits reinforce each other in certain directions only.

Question 34

What is the central beam, emitted by the diffraction grating - in the same direction as the incident beam called?

Answer 34

zero order beam
- other transmitted beams are numbered outwards from zero order beam.

Question 35

What increases the angle of diffraction between each transmitted beam and central beam?

Answer 35

• Light of longer wavelength
• grating with closer slits

Question 36

What is the equation for finding wavelength of light being used in a diffraction grating?

Answer 36

dsinx = n x wavelength

x = angle to the normal made by the maximum 
n = order of maximum 
d = distance between the slits

Question 37

Using the diffraction grating equation what re the effects of:

• greater wavelength
• greater distance between slits

Answer 37

• Greater wavelength makes sinx bigger, so the pattern spreads out more
• D being bigger makes sinx smaller, so pattern will spread out less

Question 38

What are some applications of diffraction gratings?

Answer 38

• Line absorption spectra
• X-ray crystallography