P7.2

Question 1

1. What is the speed of waves affected by?

Answer 1

-the medium they are travelling through

Question 2

1. When will wave speed change?

Answer 2

-if a wave moves from one medium into another

Question 3

2. What does a change in the speed of a wave cause? Why?

Answer 3

-a change in wavelength
-the frequency of the waves cannot change (wave speed=frequency x wavelength)
=may cause a change in direction (refraction)

Question 4

2. What would happen if the speed decreases?

Answer 4

-the wavelength would decrease

Question 5

2. What is refraction?

Answer 5

-change in direction of a wave

Question 6

3. How can refraction of light waves be explained?

Answer 6

-a change in their speed when they pass into a different medium= refraction of light waves

Question 7

3. What happens if a light ray hits a boundary (change in medium) straight on?

Answer 7

• slows, same direction

- same frequency, shorter wavelength

Question 8

3. What happens if a light ray hits a boundary (change in medium) at an angle?

Answer 8

-part slows while the other part is still going fast= change direction

Question 9

4. What do convex/converging lenses use refraction for?

Answer 9

-to focus light waves to form an image of an object

Question 10

4. What happens in a converging lens?

Answer 10

-light ray hits surface
=air to glass= slows= bends towards the normal
-light comes out of lens
=glass to air= speeds up= bends away from normal
-curvature of lens= parallel rays bend towards focal point= image is formed

Question 11

4. What is the normal?

Answer 11

-the line at right angles to the boundary at the point where the ray enters or leaves

Question 12

7. What shape is a converging lens?

Answer 12

• it is convex

- fatter towards the middle

Question 13

7. What do converging lenses do to rays of light?

Answer 13

-cause them to converge at a focus

Question 14

7. What is a principal axis?

Answer 14

-a line which passes straight through the lens

Question 15

7. What is the focal point of a lens?

Answer 15

-where rays initially parallel to the principle axis meet

Question 16

7. Where are focal points positioned?

Answer 16

-in front and behind the lens

Question 17

7. What is the focal length?

Answer 17

-the distance between the middle of the lens and its focal point

Question 18

7. What is the relationship between focal length and power?

Answer 18

-shorter focal length= converges more strongly= more powerful

Question 19

7. How do you make a more powerful lens from the same material?

Answer 19

-make it with a more strongly curved surface

Question 20

8. What is the unit of lens power?

Answer 20

-D, dioptres

Question 21

8. What is focal length measured in?

Answer 21

-m, meters

Question 22

9. How does light from astronomical objects reach the earth?

Answer 22

-as effectively parallel sets of rays

Question 23

9. Why does light from astronomical objects reach the earth as effectively parallel sets of rays?

Answer 23

-the astronomical objects are so distant

Question 24

10. What does a simple optical telescope have?

Answer 24

-two converging lenses of different powers, with the more powerful lens as the eyepiece

Question 25

10. How is the eyepiece lens made more powerful?

Answer 25

-it is more curved

Question 26

10. How is a simple optical telescope set up?

Answer 26

-lenses aligned on the principal axis so focal point the same

Question 27

11. What is a telescopes two optical elements?

Answer 27

- objective lens or mirror | - an eyepiece

Question 28

11. What is the purpose of the objective lens or mirror?

Answer 28

-collect light from the object being observed and form an (REAL) image of it

Question 29

11. What is the purpose of the eyepiece?

Answer 29

-produces a magnified image (VIRTUAL) of the image from the objective that we can view

Question 30

13. What do most astronomical telescopes have as their objectives?

Answer 30

-concave mirrors instead of converging lenses

Question 31

13. Why do most astronomical telescopes concave mirrors instead of converging lenses as their objectives?

Answer 31

- smoother so less distorted images - can reflect more types of em radiation - mirrors reflect rays of colours the same way

Question 32

14. How do concave mirrors bring a parallel beam of light to a focus?

Answer 32

-parallel rays of light shine on concave mirror, then reflect and converge

Question 33

15. Why are large telescopes needed to collect the weak radiation from faint or very distant sources?

Answer 33

- need a huge objective lens or mirror to collect enough radiation as little radiation reaches us - diameter of objective lens= aperture - bigger aperture= more radiation= better image formed

Question 34

15. What is an aperture?

Answer 34

-diameter of an objective lens

Question 35

16. What is diffraction?

Answer 35

-when waves spread out from a narrow gap

Question 36

18. What can be diffracted?

Answer 36

-light

Question 37

18. When is the effect of diffraction on light most noticeable?

Answer 37

-when light travels through a very small gap, comparable to the wavelength of the wave

Question 38

18. Under what circumstances is a wave diffracted most?

Answer 38

-narrower gap or longer wavelength

Question 39

19. What size must the aperture of a telescope be?

Answer 39

-aperture must be larger than wavelength of the radiation detected by the telescope= sharp images

Question 40

19. What happens if the size of the aperture of a telescope is smaller than the wavelength of radiation detected?

Answer 40

-diffraction causes images to blur

Question 41

20. How can a spectrum be produced by refraction in a prism?

Answer 41

- white light disperses as it enters the prism - different wavelengths of coloured light refract by different amounts - boundaries aren’t parallel so different wavelengths don’t recombine like they would in a rectangular prism= rainbow not white light

Question 42

20. What is white light?

Answer 42

-a mixture of lots of wavelengths of coloured light

Question 43

21. How can a spectrum be produced?

Answer 43

-by a diffraction grating

Question 44

21. What does a diffraction grating produce?

Answer 44

-a spectrum

Question 45

21. What is a diffraction grating?

Answer 45

-has very narrow slits that diffract light

Question 46

21. How does a diffraction grating make a spectrum?

Answer 46

- light passes through gaps | - different wavelengths of coloured light diffract by different amounts= spectrum of coloured light

Question 47

21. What are spectra from a diffraction grating used for?

Answer 47

-to analyse light coming from stars