Answers to optics questions
1)
a) C
b) Light travels in straight lines. A straight line from the source
passing close to the puppet’s foot will hit the screen at point C.
c) Light travels VERY fast.
2)
2 Light travels from the boy in straight lines. Some
rays go through the pin-hole into the camera. The rays
make a picture on the screen called an image. The image is
upside-down and very faint. You can make the image brighter
by making the pin-hole bigger, but then it gets blurred.
3)
On a microscopic scale, the surface of paper is rough whereas
the surface of the mirror is smooth. Therefore light hitting the
mirror is scattered (reflected in all directions) while light
hitting the mirror obeys the law of reflection (Ii= r).
4)
D
5)
a) incident ray, normal, reflected ray
b) 50°
6)
At a plane mirror, the angle of incidence is equal to
the angle of reflection. The distance from the object to the mirror
is equal to the apparent distance from the mirror to the
image.
7)
a) i) We did this yesterday.
ii) After being reflected twice, the reflected ray should be
parallel to the incident ray.
b) The light ray is reflected back towards the source of the light.
8)
i) At night the light of the headlights is reflected back
towards the car and thus into the driver’s eyes.
ii) A plane (flat) mirror
would not reflect the light back towards the source unless the light
happened to arrive along the normal.
9)
Light travels from the boy in a straight line. Some
rays go into the periscope. The rays are reflected from the
top mirror down the periscope into the bottom mirror.
The light rays are reflected from the bottom mirror into the
person’s eye. The light rays form an image at the back of the
person’s eye.
CURVED MIRRORS WILL
NOT BE ON THE TEST
10)
A concave mirror would give a magnified view of your
face while a plane mirror would give you a ‘normal’, non-magnified
view. A convex mirror would allow you to see more (a wide
angle reflection).
11)
In the convex mirror everything would appear small and
distant but the mirror would $reflect objects across a wide angle.
The concave mirror would make everything appear upside-down
(your face does not appear upside down in a shaving mirror since
your face is too close to the mirror for this effect to happen).
12)
It would be unhelpful to see the traffic reflected upside
down!
13)
Plane
mirrors can be found in bathrooms, for shaving, in bedrooms, for
dressing, in cars, for rear view.
Concave
mirrors can be found in bathrooms, for a magnified view while
shaving, in torches and spotlights and searchlights, to focus the
light, in large astronomical telescopes, to focus the light.
Convex
mirrors can be found in car rear-view wing mirrors, for a wide angle
of view, in supermarkets, for a wide angle security view, in awkward
locations on roads to give drivers exiting driveways a wide angle
view of approaching traffic.
14)
The light from the fish is bent (refracted) when it passes
from the water into the air (via the glass). It bends away from the
normal, so the refracted rays appear to the eye to have come from a
point closer to the person’s eye than the fish really is.
15)
Light hitting the surface of a transparent material along the
normal (at right angles to the surface) is not refracted – it
continues straight along the normal.
16)
Other examples: a prism can bend light (and disperse light
into a spectrum). Swimming pools look less deep than they really
are. Lenses make use of refraction.
17)
a) Towards the normal, then away (parallel with incident
ray) b) same as (a)
c) Goes straight d) same as (a)
18)
b) The red filter blocks all colours except red. In terms of
the primary colours, the filter blocks green and blue and allows red
to pass.
19)
If yellow light mixed with blue light hits a white screen the
colour seen is white. This happens because yellow light
stimulates the green and red sensors in the retina (yellow light can
be said to be made up of red + green) and the presence of blue light
means that all three primary colours are present, which together
make up white light.
20)
White light passes through a yellow filter, and then a cyan
filter. The colour on the screen will be green because the
yellow filter transmits red and green and the cyan filter transmits
the green light (a cyan filter is also capable of transmitting blue
light but the blue light had already been blocked by the yellow
filter).
21)
A red dress will appear red in yellow light since the
yellow light consists of red and green light. The red dress is
capable of reflecting only red light and since that colour is
present in the yellow light it will be reflected. In green light it
will appear black because the green light does not contain
any red light for the dress to reflect. The red dress absorbs any
green or blue light which is present.
22)
a) Looking through the red lenses, the red flowers appear
red, the blue pot appears black and the green leaves appear black.
b) Looking through the yellow lenses, the green leaves appear green
and the red flowers appear black.
23)
a) Some of the light from the lamp hits the ball. Some of
this light is reflected by the ball. Some of the reflected light
enters Naomi’s eyes and her eyes focus the light on her
light-sensitive retina allowing her to see the ball.
b) i)
