# KS4 Waves : Reflection.

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KS4 Waves : Reflection

Objectives Recall that light travels in straight lines.
Draw accurate ray diagrams. Describe how we see objects. Define the words opaque and transparent. Describe the images formed in a plane mirror. Recall how the angle of incidence and angle of reflection are related for a plane mirror. Describe the differences between convex and concave mirrors.

Light rays A light ray is a narrow beam of light. When drawing light diagrams always….. 1. Use a ruler. 2. Add an arrow to show the direction the light ray is travelling in.

How does light travel? 1) Fill a clear glass trough or empty fish tank with smoke. 2) Shine rays of light through the tank and describe what you see.

Light travels in straight lines
Your experiment shows that light travels in straight lines. What every day consequences do you think this fact has? You can not see around corners. Shadows are formed in the area behind an object where light can not reach.

Seeing objects How do we see an object?
There are two ways we see objects….. Do we see all objects in the same way? We see some objects because they are light sources. We see some objects by reflected light.

Objects seen by reflected light
Seeing objects Light sources Objects seen by reflected light Sun Jupiter Firefly Moon Light bulb Book Light bulb Sun Jupiter Book Firefly Moon

Light sources How do light rays from the Sun and other light sources reach our eye? They travel in a straight line directly into our eye.

Reflected light How do light rays from a book and other such objects reach our eye? Light from the Sun or another light source strikes the book and some of the light is reflected into our eye.

Opaque and transparent materials
Use a textbook or other source to find out what the words opaque and transparent mean. Opaque materials do not let light though. Transparent materials do let light through.

Opaque and transparent materials
Hold different materials between a lamp and a screen. Material More or less opaque? Coal Water Glass Thin ice Coffee more less less less Fill in the results table shown estimating if the materials are more or less opaque than vegetable oil. more

There are two types of m____. F__ mirrors and c____ mirrors.
Reflections There are two types of m____. F__ mirrors and c____ mirrors. Flat mirrors are known as p___ mirrors. If a mirror curves in then it is a c_____ mirror, if it curves out it is a c____ mirror. irrors lat urved lane oncave onvex

Plane mirrors Write out the letters of the alphabet down one side of a piece of paper, then next to the letter draw what the letter would look like as seen in a mirror. What do you predict the letter ‘L’ would look like in a mirror? What do you predict the letter ‘P’ would look like in a mirror?

Right and left What happens to writing in a mirror?
The writing in a mirror is laterally inverted or back to front. If you are wearing a T-shirt with some writing on and wave your right hand what will your image appear like in a mirror? The writing on your T-shirt will be back to front and your image will be waving it’s left hand.

Ambulance Ambulance Rear-view mirrors
Imagine you are driving along in your car…… Ambulance …and then you look up and see the following in your mirror….. Ambulance …with your knowledge of mirrors how must the writing be written on the vehicle?

Reading in Mirrors  In small groups, take it in turns to read the list of words on the next slide with your back to the screen using a mirror.  You can only move on to the next word when you read the first word correctly.  Put your results in a table like this:

Dog Man Ball Bat Bike Ants Park Fins Pink Litter Sandy Shark
Words Dog Man Ball Bat Bike Ants Park Fins Pink Litter Sandy Shark No Cheating

Results A) Who read the words in the quickest time?
B) Plot a bar chart of you r results like: C) What was the average time taken in your group?

Image distance I 1. Fix a plane mirror along the centre of a piece of A4 paper and draw around it. Place a pin as the object in front of the mirror. 2. Line up a ruler with the image of the pin and draw along the edge of the ruler on the paper. Repeat for 3 more positions of the ruler. 3. Remove the mirror and ruler. Where the lines cross is the image position. What are the distances between the mirror and object and its image?

Image size and distance
For a plane mirror how does the size of the image and the image distance from the mirror relate to the original object? Image Object Image size and object size are the same. The image distance is the same as the object distance from the mirror.

Investigating reflection
Apparatus: Power pack Ray box Slit Plane mirror Protractor Ruler The ray box will get very hot, be careful when handling it.

Investigating reflection
1. Place the plane mirror on a piece of paper. Incident ray 2. Draw a line across the front of the mirror. Incident angle i 3. Draw a normal to the mirror. Reflected angle r 4. Shine rays of light at varying angles to the normal and record the position of the incident and reflected rays Reflected ray 5. Measure the incident angles and the reflected angles for each ray. Record your results in a table.

Angle of incidence = Angle of reflection
Results i r 30° 45° 70° 85° 30° 45° 70° 85° Angle of incidence = Angle of reflection What do you notice? Is there a pattern?

Using plane mirrors By positioning two plane mirrors at 45° to each other at either end of a tube we can make a _________. periscope Periscopes are used in __________. submarines

ƒ Reflection : Curved Mirrors Finding  of a concave mirror.
Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ]. Hold the mirror in the other hand and move it closer to the screen until a clear image appears. Chose a distant object [to get parallel rays of light]. ƒ Hold a plain white screen in one hand.

By changing the curvature of the mirror
Reflection : How does curvature affect ƒ?  Concave mirrors reflect rays of light to a focal point.  The distance between the mirror and the focal point is called the focal length [ƒ].  Concave mirrors produce real images because the rays of light meet [unless the object is close]. By changing the curvature of the mirror  How can ƒ be changed?

ƒ Reflection : How does curvature affect ƒ?
 Take a piece of Al or stainless steel sheet and curve it slightly. ƒ  Draw around the reflector.  Shine parallel rays of light at the reflector and plot their positions.  Measure ƒ and record your results.  Carefully bend the reflector and repeat the process to see how ƒ changes with curvature.

ƒ Reflection : Convex mirrors
 The distance between the mirror and the focal point is called the focal length [ƒ]  Convex mirrors reflect rays of light away from a focal point.  Convex mirrors produce virtual images - the rays of light do not meet.

Reflection : Curved mirrors
Concave reflectors are used to focus signals from distant satellites. Convex reflectors are used to widen the field of view.

Which of the following is not a light source?
The Sun A star Traffic lights A book

Which of the following is not seen by reflected light?
Your hand Jupiter Light bulb The Moon

Which of the following is the most opaque?
Milk Sea water Air Drinking water

What do we call the area behind an object where light can not reach?