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Smooth surfaces reflect light better than rough ones.

# Physics - Reflection

This Physics quiz is called 'Physics - Reflection' and it has been written by teachers to help you if you are studying the subject at senior high school. Playing educational quizzes is one of the most efficienct ways to learn if you are in the 11th or 12th grade - aged 16 to 18.

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Waves travel in straight lines unless they are diffracted, refracted or reflected and this quiz deals with the last of those three. Reflection is completely predictable, all you need to know is the law of reflection. This simply states that 'the angle of incidence is equal to the angle of reflection'. But what are these angles and how are they measured?

The angle of incidence is the angle at which a wave hits the reflecting surface. The angle of reflection is the angle at which the wave bounces off the reflecting surface. It doesn't matter what the surface is or how smooth or bumpy it is, the law of reflection is ALWAYS obeyed.

This is where it gets tricky. The angle is not measured from the surface, it is measured from the normal. The normal is an imaginary line at right angles to the reflecting surface where the ray hits. When you have been doing reflection experiments at school you will have probably used ray boxes and plane mirrors. When you have traced out the path the light rays took you will have made this imaginary line real by drawing it so that you could measure the angles.

Smooth surfaces produce good reflections and can act as mirrors when light waves hit them. This can happen with other waves from the electromagnetic spectrum. You can feel the infrared radiation from the sun by reflecting sunlight onto the back of your hand using a small, flat mirror. WARNING: if you do try that, be very, very careful not to reflect the sunlight anywhere near your eyes as it ould easily damage them.

Longitudinal waves can be reflected too. They follow exactly the same rule of reflection as electromagnetic waves. Sound waves are longitudinal and when a sound wave is reflected from a cliff face or the wall of a building, you hear the reflection as an echo.

1.
Electromagnetic radiation travels in straight lines, so how come certain radio waves can be detected on the other side of the world without using artificial satellites?
Earth's gravity causes them to bend round
They reflect off a layer in the Earth's atmosphere
They can pass directly through the Earth
When they get to the edge of the atmosphere they are trapped by space
The reflecting layer is called the ionosphere
2.
A solar furnace can be used to cook food in sunny places where it is difficult to obtain other fuels. It works by reflecting and focussing the Sun's heat. What would be the best shape and material for a solar furnace?
Flat, dark and matt
Concave, light colored and shiny
Convex, light colored and matt
Flat, gray and shiny
Since it needs to focus the Sun's thermal radiation (infrared) onto a cooking vessel, the only option is concave. The others all produce vurtual images
3.
Which of the following statements about reflection is not true?
Echoes only occur in the countryside
The angle of incidence is the same as the angle of reflection
The normal is drawn as a dotted or dashed line on ray diagrams
Rough surfaces scatter sound in all directions
Echoes occur in towns and cities too but you are not usually aware of them
4.
What is a virtual image?
An image that can be projected onto a screen
An image that can't be projected onto a screen
An image that is virtually perfect
An image of a person only
Plane mirrors give virtual images, they cannot be projected onto a screen. Concave mirrors give real images, they can be reflected and seen on a screen
5.
A sound wave hits a brick wall at 36o from the normal. At what angle does it reflect?
126o
63o
54o
36o
Remember the Law of Reflection...
6.
When you look at yourself in a mirror, your image is:
real and identical to you
virtual and identical to you
real and laterally inverted
virtual and laterally inverted
Right and left are reversed in a reflection
7.
The location of a virtual image in a plane mirror appears to be where?
The same distance behind the mirror as the object is in front
The same place as the object
Behind the mirror but at half the distance of the object
Behind the mirror but at twice the distance of the object
A reflection always appears to be as far behind the mirror as the image is in front. If you haven't already learned about Pepper's Ghost in your science lessons, look it up when you have finished on this website
8.
On ray diagrams, what is the normal?
An imaginary line at 90o to the reflecting surface
An imaginary line at 45o to the reflecting surface
A real line at 90o to the reflecting surface
A real line at 180o to the reflecting surface
It is drawn from the point at which the incident ray hits the reflecting surface
9.
What is an incident ray of light?
A ray of light that the police shine when there has been an incident on the road
A ray of light that is leaving the surface of a mirror
A ray of light that is hitting the surface of a mirror
A ray of light that is shining parallel to the surface of a mirror
You can also use the phrases 'incident wave', 'incident wavefront', 'incident sound wave' etc. In terms of waves, incident simply means the wave that is approaching the reflecting surface
10.
A hiker shouts to their friend and 4 seconds later hears an echo of their shout. If sound travels at 330 m/s, how far away was the cliff that reflected their shout?
At least 2 km