Lenses & Angles

Question 1

What is the formula (with units) for calculating power?

Answer 1

Power (dioptres, D) = 1/Focal Length (always in metres)

Question 2

Why is a minus sometimes shown to show the power?

Answer 2

Because it is for a diverging lens

Question 3

Give the formula used to calculate the image lenses by a lens

Answer 3

1/f = 1/u + 1/v
f being focal length
u being object distance
v being image distance

Question 4

What are “real images?”

Answer 4

Images that can be focused into a screen

Question 5

What are “virtual images?”

Answer 5

An image that cannot be formed on a screen

Question 6

In a lens equation, when v (image distance) is negative, what does this mean?

Answer 6

This means the image is virtual and cannot be formed on a screen

Question 7

What is the law of reflection?

Answer 7

Angle of Incidence = Angle of Reflection

angle i = angle r

Question 8

What is refraction?

Answer 8

It is the change in the direction of a light ray that occurs when it travels from one transparent material into another

Question 9

How does light bend when entering and leaving glass?

Answer 9

1) From air to glass, the ray moves towards the normal line

2) From glass to air, the ray moves away from the normal line

Question 10

What happens when rays of light meet a surface at 90 degrees?

Answer 10

They do not bend at all but simply continue into the material with direction change

Question 11

Do light waves travel more slowly in glass than they do in air?

Answer 11

Yes, they do

Question 12

What changes when waves enter a denser material like glass?

Answer 12

They travel more slowly and as they travel, the direction is turns through an angle. This angle depend on how much the wave had slowed down. Wavelength also changes.

Question 13

What is the law of refraction?

Answer 13

Snell’s Law - n = sin i/sin r

Question 14

When does total internal reflection occur?

Answer 14

This occurs when light travelling from a dense material like glass meets a boundary with a less dense material like air

Question 15

Total internal reflection can take place with what else other than light?

Answer 15

Sound

Question 16

What formula is used to calculate the critical angle?

Answer 16

sin c = 1/n

Question 17

What does the value of the critical angle depend on?

Answer 17

It depends on the refractive index of the material. The bigger n is, the smaller the value of the critical angle

Question 18

What angle must me larger than the critical angle in order for total internal reflection to occur?

Answer 18

The angle of incidence

Question 19

What are optical fibres?

Answer 19

Thin glass fibres that are used to transmit digital data in the form of pulses of lights

Question 20

How do optical fibres all light to travel?

Answer 20

The light travels along the fibres by total internal reflection

Question 21

How are optical fibres used in endoscopes in keyhole surgery?

Answer 21

Endoscopes have two optical fibres. One carries light to the location the surgeon needs to look at. The other has a lens to view the image. Light from the lens travels back along the second optical fibre to the surgeons eye.

Question 22

What is “ultrasound?”

Answer 22

It is a sound with a much higher frequency than can be heard by humans

Question 23

How do ultrasounds produce images?

Answer 23

Ultrasound can travel through human tissue without harm. Some ultrasound is reflected whenever there is a change in the speed of the ultrasound waves. A computer builds up an image from the reflected and refracted waves.

Question 24

What is “f” in terms of lenses?

Answer 24

f is the focal length.