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7.3Chapter 7: Light

Reflection and images — Alhussan Physics - Stage 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 12 1 quiz

📚 Alhussan Physics - Stage 8 Hub / Topical Revision
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Lesson summary

Physics 7.3 Reflection and images — Plane Mirrors, Virtual Images and Periscopes — Alhussan International Academy — Physics (Stages 7–9) notes: Use the law of reflection to explain how images form and describe the types of image and reflection.Plane mirror = a flat mirror.Image p…

  • Image properties
  • Lateral inversion
  • Brain assumption
  • Ray diagram
  • Steps in forming the image
  • Top and bottom

Section 1 of 8The image in a plane mirror

Plane mirror
Plane mirror a flat mirror.
Image properties
  • The image is upright, the same size as the object and the same distance behind the mirror as the object is in front.
  • If you move closer to the mirror, the image moves closer.
  • If you move away, the image moves away.
Lateral inversion
  • The left and right of the image are swapped, but top and bottom are not.
  • This is why AMBULANCE is written backwards on the front of the vehicle, so it reads correctly in a driver's mirror.

Section 2 of 8Types of image

TypeMeaningExample
VirtualYou must look in the mirror to see it; it cannot be projected onto a screenImage in a plane mirror
RealIt can be projected onto a screen, with no need to look through anythingImage on a cinema screen
UprightThe right way upPlane mirror image
InvertedUpside downImage on a camera film or retina

Section 3 of 8How an image is formed

1Brain assumption
  • You see an image because your brain assumes that light travels in straight lines, even when it has been reflected.
2Ray diagram
  • A ray diagram is a model that uses two chosen light paths to work out where the image is.
3Steps in forming the image
  • Light is emitted by an object and spreads out.
  • Some light is reflected from the mirror.
  • The brain works out where the light appears to come from if it travelled in a straight line.
  • This is where the image seems to be, behind the mirror.
4Top and bottom
  • Light from the top of your face appears to come from the top of the image, so the image is the right way up.

Section 4 of 8Constructing a ray diagram

1
Mark the object and draw the mirror line.
2
Find the image point the same distance behind the mirror as the object is in front, on a line at 90° to the mirror.
3
Join the image point to the eye with a straight line; where it crosses the mirror is the point of reflection.
4
Draw the incident ray from the object to that point and the reflected ray to the eye, with arrows.
5
Draw the part behind the mirror as a dotted line.

Section 5 of 8Regular and diffuse reflection

Regular reflection
  • A smooth surface reflects light in a regular way and gives a clear image.
  • An image on water is clear only if the water is flat.
Diffuse reflection
  • A rough surface scatters light in many directions, so no image forms.
  • White paper reflects most light that hits it, but no image is seen because the reflection is diffuse.
All surfaces
  • Rough or smooth, every surface reflects some light.

Section 6 of 8The periscope

The periscope
Structure
  • A periscope has two parallel plane mirrors, each at 45° to the tube.
Action
  • Light from an object hits the top mirror, is reflected down the tube, hits the lower mirror and is reflected into the eye.
Uses
  • It lets you see over crowds or from a submarine.
Reflective materials
  • Reflective clothing and road studs keep people safe at night.

Section 7 of 8Key terms

TermMeaning
Plane mirrorA flat mirror.
Virtual imageAn image that cannot be projected onto a screen.
Real imageAn image that can be projected onto a screen.
Lateral inversionThe swapping of left and right in a mirror image.
Ray diagramA model showing the paths of light rays to find the position of an image.
PeriscopeA tube with two parallel mirrors used to see over obstacles.

Section 8 of 8Exam technique

1
Thinking the image in a plane mirror is real — it is virtual and cannot be projected.
2
Thinking the image is bigger or smaller than the object — it is the same size.
3
Thinking only mirrors form images — any smooth surface such as still water can.
4
Thinking diffuse reflection breaks the law of reflection — each ray still reflects with equal angles.

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