Light

Question 1

how are light rays drawn

Answer 1

straight line with an arrow to indicate direction of travel

Question 2

what are the 3 types of beams of light

Answer 2

parallel, convergent, divergent

Question 3

why are shadows formed

Answer 3

light travels in straight lines and cannot bend around an opaque object and reach behind it

Question 4

what is an umbra

Answer 4

totally dark and sharp shadow as no light reaches behind the object

Question 5

what is a penumbra

Answer 5

partial shadow due to some light reaching behind the object

Question 6

when does a lunar eclipse occur

Answer 6

when Earth moves directly between Sun and Moon (only on a night of full moon)

Question 7

when does a solar eclipse occur

Answer 7

when Moon moves directly between Sun and Earth

Question 8

what is the difference between transparent, translucent and opaque materials

Answer 8

transparent materials allow most of the light to pass through

translucent materials allow some light to pass through

opaque materials do not allow light to pass through

Question 9

what is the incident ray

Answer 9

light ray striking surface of object

Question 10

what is the point of incidence

Answer 10

point at which the light is in contact with surface

Question 11

what is the reflected ray

Answer 11

light ray that bounces off mirror

Question 12

what is the normal

Answer 12

imaginary line perpendicular to surface at point of incidence

Question 13

what are the 2 Laws of Reflection

Answer 13

angle of incidence is equal to angle of reflection

incident ray, reflected ray and normal at point of incidence all lie on same plane

Question 14

3 points

describe a regular reflection

Answer 14

when a parallel beam of light falls on a smooth surface, the reflected rays are parallel

reflections from smooth surfaces called regular or specular reflections

image formed is clear and undistorted

Question 15

3 points

describe an irregular reflection

Answer 15

when a parallel beam of light falls on a rough surface, the reflected rays are scattered in different directions

reflections from rough surfaces called irregular or diffused reflections

no images or blurred images produced

Question 16

5 points

describe an image formed by a plane mirror

Answer 16

image is:
1) upright
2) of the same size as object
3) at the same distance behind mirror as object is from mirror
4) virtual (cannot be reflected onto a screen behind the mirror)
5) laterally inverted

Question 17

what are concave mirrors

Answer 17

they have a surface that bends inwards and form virtual and magnified images if object is sufficiently close to the mirror

Question 18

what are convex mirrors

Answer 18

they have a surface that bends outwards and form images that are upright, distorted and smaller than object

Question 19

what is refraction

Answer 19

bending of light when it travels from one transparent medium to another transparent medium

Question 20

what is the refracted ray

Answer 20

light ray which is refracted

*bends more towards normal if light travels slower
*opposite side of normal as incident ray

Question 21

what is the emergent ray

Answer 21

light ray that bends away from the normal as it emerges from a medium back into the air

*parallel to incident ray

Question 22

what happens when the incident ray is perpendicular to surface of a medium (travelling along normal)

Answer 22

refraction will not take place, even though there is slowing down of light as it travels in the medium

Question 23

objects appear higher in water than it actually is in our eyes
what is this perceived depth called

Answer 23

apparent depth

Question 24

what is the refractive index

Answer 24

ratio of the speed of light in a vacuum (c) to the speed of light in a medium under consideration (v)

*has no units

Question 25

what are 2 formulae to calculate refractive index

Answer 25

n = speed of light in vacuum (c) / speed of light in medium (v)

n = sin i / sin r, where i is the angle of incidence, r is the angle of refraction

*i will always be bigger than r and i is always in the air
*n must always be larger than 1

Question 26

what is the Principle of Reversibility

Answer 26

a light ray will travel along the same path if its direction of travel is reversed

*applies to reflection and refraction of light

Question 27

what is dispersion

Answer 27

the splitting of white light (sunlight) into colours of the rainbow when passing through a glass prism

*red slows down the least, violet slows down the most
*emergent rays bend again but in a different direction (opposite to incident rays)

Question 28

what are the 7 colours of the rainbow

Answer 28

red, orange, yellow, green, blue, indigo, violet (ROYGBIV)

*this band of 7 colours called the spectrum of white light

Question 29

what is a colour filter

Answer 29

any plastic or glass of a specific colour that only lets through that part of the coloured visible spectrum and the remaining part is absorbed

*if red and blue colour filters used together, no light passes through

Question 30

what are the 3 primary colours of light

Answer 30

red, green, blue (RGB)

*can be mixed to produce white light

Question 31

what are the 3 secondary colours of light

Answer 31

red + blue = magenta
red + green = yellow
blue + green = cyan

Question 32

what colour does a blue object appear as under magenta light

Answer 32

blue (reflects blue, absorbs red)