Name: ANSWERS
Waves Workbook 2
Year 12 Science
Ruawai College 2014
Reviewing Reflection and Refraction
1
f
= 1 +
do
1
di
(you are given this formula)
1.
What is f?
focal length
2.
What is do?
distance of object from mirror / lens
3.
What is di?
distance of image from mirror / lens
m =
hi
ho
=
di
do
4.
What is m?
magnification
5.
What is hi?
height of image
6.
What is ho?
height of object
(you are given this formula)
7.
An object 2.0 cm high is placed 6.0 cm in front of a convex lens which has a focal length, 4.0 cm.
a.
State the nature of the image.
Inverted, real and enlarged
b.
Calculate the position and size of the image.
4.0 cm high
12 cm from the lens
8.
An object is placed 20 cm from a concave lens. The focal length of the lens is 15 cm.
Calculate the distance from the image to the lens.
-8.6 cm
9. An object is placed 50 cm from a concave lens which produces a virtual image at a distance of 10 cm
in front of the lens.
a.
Calculate the focal length of the lens.
-12.5 cm
b.
Draw a diagram to show the formation of the image.
10. An object of height 4.0 cm is placed at a distance of 10 cm from a convex lens of focal length 20 cm.
Find the position, nature and size of the image.
Distance between object and the lens = -10 cm
Distance between image and lens = – 20 cm
Height of image = 8 cm
The image is formed on the same side as the object and is erect, virtual and is formed at a distance of
20 cm in front of the lens
11. Draw ray diagrams to show the position and nature of the image formed by a convex lens when the
object is placed:
a.
at 2F
b.
between F and 2F
c.
beyond 2F
12.
Using a concave mirror, when a ray is parallel to the principal axis it reflects through
the focus.
13.
Using a concave mirror, when a ray passes through the focus it reflects parallel to
the principal axis .
14.
Using a convex mirror, when a ray is parallel to the principal axis it reflects off as if it
has come from the focus.
15.
Using a convex mirror, when a ray is aimed at the focus it reflects off parallel to the
principal axis .
16.
The angle of incidence equals the angle of reflection.
17.
Using a convex lens, when a ray is parallel to the principal axis it will converge on the
focus after refraction through the lens.
18.
Using a concave lens, when a ray is parallel to the principal axis it will appear to
diverge from the focus after refraction through the lens.
19.
Using either type of lens, a ray passing through the optical centre will travel on
without changing direction .
20.
Convex lenses produce real and virtual images.
Concave lenses only produce virtual images.
Refraction at a Medium Barrier
Refraction is the change in velocity of light as it changes medium. Light changes speed and
direction when it changes medium.
A light ray bends towards the normal as it enters a denser medium.
21.
Which is denser, air or water?
water
22.
Do light rays bend toward the normal or away from the normal when the go from
water to air?
away from the normal
23.
Draw the normal on the diagram to show how the ray of light bends away from the
normal when it goes from water to air / dense to less dense.
Line that is 90o to the boundary
24.
How would you describe the normal?
25.
Is the angle of incidence greater than or less than the angle of refraction? greater
26.
Is Medium 1 less dense or more dense than Medium 2?
27.
A light ray bends towards the normal when it enters a more dense medium.
less dense
28.
Is air more dense or less dense than glass?
Less dense
29.
What happens to the light ray as it enters the glass from the air?
bends towards normal
30.
What happens to the light ray as it enters the air from the glass?
bends away from normal
31.
Which is denser – glass or water?
32.
Explain your answer to 31.
than in water
Glass
Refracted ray bends more towards normal in glass
The refractive index (n) of a medium indicates how much the velocity (speed and direction)
of light rays changes when they enter the medium.
The relative refractive properties of two media is described using Snell’s formula / law.
33.
What is the name of the law that describes the relative refractive properties of two
media?
Snell’s law
34.
What symbol describes the refractive index of medium 1?
n1
35.
What symbol describes the refractive index of medium 2?
n2
36.
What symbol describes the velocity of light in medium 1?
v1
37.
What symbol describes the velocity of light in medium 2?
v2
38.
What symbol describes the angle of incidence?
ϴ1
39.
What symbol describes the angle of refraction?
ϴ2
40.
What is formula of Snell’s Law?
n1.sin ϴ1= n2.sin ϴ2
(you are given this formula you do not have to remember it)
41.
Rearrange the formula to find the refractive index of medium 1.
n1. = n2.sin ϴ2
sin ϴ1
42.
Rearrange the formula to find the refractive index of medium 2.
n2 = n1.sin ϴ1
sin ϴ2
Sometimes the refractive index is also called the index of refraction.
43.
What is the n1?
refractive index of medium 1
44.
What is the value of n1?
1.00
45.
What is ϴ1?
angle of incidence
46.
What is the value of ϴ1?
24.5o
47.
What is ϴ2?
angle of refraction
48.
What is the value of ϴ2?
16.7o
49.
What is sin ϴ1?
sine function of 24.5
50.
What is the value of sin ϴ1?
0.4147
51.
What is sin ϴ2?
Sine function of 16.7
52.
What is the value of sin ϴ2?
0.2874
Exercises:
53.
Light travels from one type of plastic (n1 = 1.8) into another plastic (n2 = 1.2), at an
angle of incidence ϴ1 = 25o.
a.
Calculate the angle of refraction - ϴ2.
39o
b.
Is the angle of refraction greater than or less than the angle of incidence?
Greater than – refractive index has reduced
54.
Light travels from water (refractive index = 1.33) into glass (refractive index = 1.52),
at an angle of incidence of 30o.
a.
Calculate the angle of refraction.
26o
b.
Is the angle of refraction greater than or less than the angle of incidence?
Less than
c.
Explain your answer to b.
refractive index has increased
55.
Light travels from glass into water at an angle of incidence of 30o.
a.
Which is the incident medium?
b.
Calculate the angle of refraction.
glass
35o
c.
Is the angle of refraction greater than or less than the angle of incidence?
Greater than
d.
Explain your answer to c.
refractive index has reduced
When crossing the boundary the frequency of waves does not change – but the speed and
wavelength do – so speeds are in the same ratio as that of their wavelengths.
λ1
λ2
=
v1
v2
Refraction is commonly noticed when light travels from air into water or glass. The change
in direction is the visible feature; the change in speed is not.
56.
What does the pencil in water look like?
Looks like it is broken / bent
57.
Explain what has happened to the light to make the pencil appear this way.
Appears broken because the light has changed direction
Putting all we know about refraction together:
λ1
λ2
=
v1
v2
=
sinϴ1
sinϴ2
=
n1
n2
The refractive index of a vacuum is exactly 1. Air has a refractive index of 1 because the
speed of light in a vacuum and in air is almost the same.
In the diagram at the top of the page, ϴ1= 42o and ϴ2= 30o , the refractive index of water
is:
nwater = sinϴ1 = 1.33
sinϴ2
58.
What is the value of ϴ1?
42o
59.
What is the value of sinϴ1?
O.669
60.
What is the value of ϴ2?
30o
61.
What is the value of sinϴ2?
0.5
63.
Where does the 1.33 come from?
0.669 ÷ 0.5
The speed of light in air is 3.00 x 108 m s-1
Exercise:
1.
The refractive index for a plastic is 1.4. Light travels from air into this plastic.
a.
Calculate the speed of light in the plastic.
2.14 x 108 m s-1
The light now travels from this plastic into water.
b.
Calculate the speed of light in water.
2.26 x 108 m s-1
c.
When the light goes from plastic into water, will it speed up or slow down?
Light will speed up
d.
Explain your answer.
Optical density of water is less than plastic