Optics

Question 1

State the characteristics of a coherent light source:

Answer 1

same frequency and same wavelength and a fixed phase difference e.g lasers

Question 2

Describe Young’s Double slit experiment:

Answer 2

Demonstrates interference of two coherent light sources:
Made by using a single slit (fixes path difference) then two slits and filter to make light monochromatic.
Slits should be similar size to wavelengths for better diffraction
Light fringes (constructive) and dark fringes (destructive) are formed.

Formula: W = lander * D / s
W = fringes spacing D = distance between screen and slits s = slit seperation

Question 3

Why would you use a white light instead of monochromatic light in Young’s Double slit experiment

Answer 3

Gives wider maxima and a less intense diffraction pattern with a central white fringe. Violet closest to central maxima and red the furthest.

Question 4

What was Young’s experiment evidence for?

Answer 4

Wave nature of light because diffraction and interference were wave properties.

Question 5

What is diffraction? and how does slit size affect this.

Answer 5

It is the spreading out of waves when they pass through a gap.
Greatest diffraction occurs when the gap is the same size of the wavelength
When the gap is smaller than wavelength most of the wave is reflected
The wider an obstacle the less of a diffraction there is

Question 6

What does monochromatic light look like on a screen?

Answer 6

Bright central fringe which is double the width of all other fringes with alternating dark and bright fringes.
The intensity of fringes decreases away from central fringe.

Question 7

With white light when diffracted which colour is furthest away from the central maxima and which colour is the closest

Answer 7

Closest - violet
Furthest - red

Question 8

What happens when you increase the slit width.

Answer 8

Diffraction decreases, central maxima becomes narrower and becomes more intense.

Question 9

What happens when you increase the wavelength?

Answer 9

Diffraction increases, central maxima becomes wider and intensity decreases.

Question 10

What is a diffraction grating

Answer 10

It is a slide containing many equally spaced slits very close together
Central maxima becomes more narrower the more slits there are

Question 11

What is the formula for diffraction grating

Answer 11

d * sin = n * lander
d distance between slits
Theta angle to the normal made by maxima
n is the order

Question 12

What happens to the diffraction grating when you increase wavelength (e.g blue to red light)

Answer 12

• the distance between the orders would increase because theta is larger due to increase in diffraction as slit spacing is closer in size to the wavelength. So the pattern would spread out.

Question 13

What are the applications of diffraction gratings?

Answer 13

• split up light from stars to get line absorption spectra which can be used to determine which elements are in the stars
• X-ray crystallography, can be used to measure atomic spacing in certain materials

Question 14

What is a refractive index and how can it be calculated using speed?

Answer 14

is a property of a material that measures how much light slows down passing through it.

n = speed of light / speed of light in substance

Question 15

What does high refractive index mean?

Answer 15

Known as being more optically dense as light doesn’t slow down significantly
(Refractive index of air is 1)

Question 16

When does refraction occur?

Answer 16

When a wave enters a different medium, causing it to change directions either towards or away from the normal depending of the materials refractive index.

Question 17

What is snell’s law?

Answer 17

n1sintheta = n2sintheta2
As light moves across the boundaries of two materials its speed changes

Question 18

How can you tell that a material is more or less optically dense than another material using a diagram.

Answer 18

Second medium is more optically dense when rays of light slows down and bends towards the normal.
Second medium is less optically dense when ray of light bends away from the normal.

Question 19

What is the term given to the angle of refraction when it is exactly 90 degrees and how do we work is out?

Answer 19

Critical angle = n2/n1

Question 20

when does total internal reflection occur?

Answer 20

When angle of incidence is greater than critical angle and the incident refractive index is greater that the refractive index of material at the boundary.

Question 21

Explain a useful application of Total internal reflection?

Answer 21

In optical fibres.
Core is optically dense
Cladding has lower optical density
Cladding protects core, prevents light from escaping and prevents signal degradation

Question 22

Signal degradation can be caused by:

Answer 22

Absorption - when parts of the signals energy is absorbed by the wire, which reduces amplitude

Dispersion - this causes pulse broadening when received signal is broader than original

Question 23

Explain the two types of dispersion that causes pulse broadening?

Answer 23

Modal - light enters the wire at different angles so they take different paths, this causes time to travel to be different (reduced by making the core narrow)

Material - caused by using light consisting of different wavelength so rays would travel at different speeds. (prevented by using monochromatic light)

(both can be reduced by using optical fibre repeaters which regenerate the signal during its travel)

Question 24

How can LED be placed so that there is no deviation of light

when entering the prism

Answer 24

LED light is entering the prism at normal to the surface

Question 25

How can you ensure in a design that there is no deviation of light rays in two different substances

Answer 25

• ensure they’re of the same refractive index
• ensure there are no gaps between the substances

Question 26

What formula do you use to work out the critical angle

Answer 26

You use snells law where one is sin90

Question 27

How can a small uncertainty in refractive index affects the path of the ray

Answer 27

The percentage is marginal so may not change as much

light travels straight