CLASS 8 SCIENCE · CHAPTER 10 · STUDY NOTES
Light: Mirrors and Lenses
Understand light: mirrors and lenses through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
EXPLORE
Identify the reflecting side of a curved mirror
“Concave” describes an inward curve; “convex” describes an outward bulge.View the surfaces from the side
- Concave mirror
- A mirror whose reflecting surface curves inward.
- Convex mirror
- A mirror whose reflecting surface bulges outward.
- Spherical mirror
- A mirror with a reflecting surface shaped as part of a sphere.
Look at the reflecting face, not the back coating. The inside of a shiny spoon is concave; the outside is convex. A spoon is a useful comparison, though it is not a perfect spherical mirror.
A plane mirror is flat. Curving the reflecting surface changes which directions the reflected rays travel, so the image can have a different size.
Take a moment. What do you think?
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Question
Which face of a shiny spoon is the concave reflecting surface?
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An image depends on the mirror and object position
Separate orientation from size.Compare orientation and size
- Erect image
- An image with the same upright orientation as the object.
- Inverted image
- An image turned upside down relative to the object.
- Diminished image
- An image smaller than the object.
A plane mirror makes an erect image of the same size. For an ordinary object, a convex mirror makes an erect, diminished image. A concave mirror can give different results at different distances.
Very close to a concave mirror, within its focal distance, the image is erect and enlarged. Farther away it can be inverted. “Concave always makes things bigger” is therefore incorrect.
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Question
A mirror gives an erect smaller image over ordinary object distances. Which curved mirror fits?
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Measure both reflection angles from the normal
The normal is perpendicular to the surface at the point hit.The normal is the angle reference
- Incident ray
- The ray arriving at a reflecting surface.
- Reflected ray
- The ray leaving the surface after reflection.
- Normal
- A line perpendicular to the surface at the point of incidence.
The angle of incidence equals the angle of reflection: i = r. Both are measured from the normal, not from the mirror surface. The incident ray, reflected ray and normal all lie in one plane.
These laws apply to curved mirrors too, but the local normal changes from point to point. If the incident ray is along the normal, i = 0° and the reflected ray returns along the same line.
- 1
Draw the mirror and mark the point hit.
- 2
Draw a normal at 90° to the mirror.
- 3
Measure the incident angle from this normal.
- 4
Draw the reflected ray on the other side at the same angle.
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Question
A ray meets a plane mirror at 30° to the normal. What angle does the reflected ray make with the mirror surface?
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A curved mirror redirects parallel light
Convergence means rays approach; divergence means rays spread.Follow the arrows back from the mirror
- Model
- Orange arrows go towards the mirror; blue arrows return towards the focus region. This is a qualitative paraxial sketch, not an exact spherical ray trace. Reflected rays remain on the incoming side of the mirror.
- Converging rays
- Rays travelling towards a common region or point.
- Diverging rays
- Rays travelling apart.
- Focus
- The point or small region where a suitably directed converging light beam meets.
A concave mirror can bring near-axis parallel rays towards a focus. A convex mirror spreads the reflected rays outward. Every ray still obeys the laws of reflection at its point of contact.
Concentrated sunlight can be dangerous to eyes and can start a fire. Use safe classroom light-ray models; never look at the Sun through optical equipment or try to burn things.
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Question
A mirror brings incoming parallel rays towards a small region. Which type is suitable?
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Choose a mirror for its job
A wide view and a close-up view need different image properties.Wide view versus close detail
- Field of view
- The area that can be seen through an optical device from a particular position.
Convex vehicle mirrors show a wider area but make images smaller. A smaller image can make a vehicle seem farther away, which explains the warning on many side-view mirrors.
A dentist may use a concave mirror close to a tooth for an enlarged view. Reflectors can also collect and direct light, as in torches and some telescopes.
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Question
Why should a driver be cautious when judging distance from a convex mirror?
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Light passes through a lens
Use middle thickness to identify ordinary glass lenses in air.Compare cross-sections
- Lens
- A transparent optical piece with at least one curved surface.
- Convex lens
- A lens thicker at the middle than at the edges.
- Concave lens
- A lens thinner at the middle than at the edges.
A mirror reflects light; a lens transmits and bends it. In air, an ordinary convex glass lens converges parallel light, while a concave glass lens diverges it.
A round water drop can behave like a lens. Its shape bends light so nearby lettering may look enlarged. The result depends on shape and viewing distance.
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Question
Which ordinary lens has thin edges and a thicker middle?
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A convex lens can magnify or form an inverted image
Object distance changes the image, even with the same lens.A converging lens transmits light
- Magnification
- A comparison of image size with object size.
With the object within its focal distance, a convex lens can act as a magnifying glass and produce an erect enlarged virtual view. Beyond the focal distance, it can form an inverted real image on a screen.
For an ordinary object in air, a concave glass lens gives an erect diminished virtual image. “Every lens magnifies” is incorrect.
Let’s look a little closer.
Go deeper: Image type matters too
A real image forms where light actually meets and can be caught on a screen. A virtual image appears to come from a place where light does not actually meet. “Erect/inverted,” “larger/smaller” and “real/virtual” answer different questions.
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Question
Why does moving a magnifying glass far from a page stop giving the same upright enlarged view?
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Lenses help us see, record and investigate
Use several applications without assuming all devices use one lens.Apparent position follows the light path
- Refraction
- A change in light’s direction when it passes between materials under suitable conditions.
Cameras form images using lens systems; microscopes make tiny features easier to observe. Glasses correct particular vision problems with suitable optical designs. The eye has a lens that changes shape to help focus at different distances.
A pencil seen partly through water can appear bent or shifted because light changes direction at material boundaries. The pencil itself has not bent.
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Question
A pencil looks bent at the water surface. Has the pencil physically changed shape?
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