Understanding Lenses and Refraction in Optics

1. A convex (converging) lens can bring parallel rays to a focus.

True

False

Convex lenses converge light. The concept is a converging lens, which bends parallel rays toward a focus.

Explanation

Convex lenses converge light. The concept is a converging lens, which bends parallel rays toward a focus.

2. Best grade 9 summary for lenses:

Lenses work by reflection

Lenses shape images by refraction at curved surfaces

Lenses change light into sound

Lenses only work in darkness

Curved refraction is the key. The concept is refraction by curved surfaces, which enables focusing and image formation.

Explanation

Curved refraction is the key. The concept is refraction by curved surfaces, which enables focusing and image formation.

3. A concave lens can help correct myopia by diverging rays before they enter the eye.

True

False

Diverging makes the image form farther back. The concept is divergence to shift focus, moving the image position onto the retina for myopia.

Explanation

Diverging makes the image form farther back. The concept is divergence to shift focus, moving the image position onto the retina for myopia.

4. A convex lens forms an image because it:

Always reflects rays

Bends rays toward the axis so they meet

Blocks half the rays

Speeds rays up in air

Convergence via refraction forms images. The concept is converging refraction, bringing rays to an image point.

Explanation

Convergence via refraction forms images. The concept is converging refraction, bringing rays to an image point.

5. Which are uses of lenses?

Glasses

Cameras

Telescopes

Measuring electric current directly

A–C use lenses; current is measured with electrical instruments. The concept is lens-based image formation, which applies to imaging and magnification devices.

Explanation

A–C use lenses; current is measured with electrical instruments. The concept is lens-based image formation, which applies to imaging and magnification devices.

Submit

6. When light slows down entering a denser medium, wavelength:

Increases

Decreases

Stays the same

Becomes infinite

v=fλ; with constant f, lower v means lower λ. The concept is the wave equation (v=fλ), linking lower speed to shorter wavelength when frequency is unchanged.

Explanation

v=fλ; with constant f, lower v means lower λ. The concept is the wave equation (v=fλ), linking lower speed to shorter wavelength when frequency is unchanged.

7. A camera focuses by using lenses that refract light to form an image.

True

False

Camera lenses form images by refraction. The concept is image formation by refraction, where lens position changes the focus on the sensor.

Explanation

Camera lenses form images by refraction. The concept is image formation by refraction, where lens position changes the focus on the sensor.

8. Long-sightedness (hyperopia) is usually corrected with a:

Convex lens

Concave lens

Mirror

Filter

Converging lenses help focus near objects. The concept is hyperopia correction, using a converging (convex) lens to increase convergence.

Explanation

Converging lenses help focus near objects. The concept is hyperopia correction, using a converging (convex) lens to increase convergence.

9. A lens bends light mainly due to:

Reflection only

Refraction at its surfaces

Magnetism

Sound

Lenses use refraction to redirect rays. The concept is refraction at curved surfaces, which changes ray directions to form images.

Explanation

Lenses use refraction to redirect rays. The concept is refraction at curved surfaces, which changes ray directions to form images.

10. Short-sightedness (myopia) is usually corrected with a:

Convex lens

Concave lens

Polarizer

Prism

Diverging lenses move the focus back. The concept is myopia correction, using a diverging (concave) lens to reduce convergence.

Explanation

Diverging lenses move the focus back. The concept is myopia correction, using a diverging (concave) lens to reduce convergence.

11. A flat glass window can still refract light, but the ray often exits:

At a random angle

Parallel to the incoming ray (shifted sideways)

Always at 90°

Only reflected

With parallel faces, entry/exit refraction cancels direction change. The concept is parallel-sided slab refraction, where the ray emerges parallel but laterally displaced.

Explanation

With parallel faces, entry/exit refraction cancels direction change. The concept is parallel-sided slab refraction, where the ray emerges parallel but laterally displaced.

12. A prism can separate white light into colours because different colours refract differently.

True

False

Dispersion: refractive index depends on wavelength. The concept is dispersion, meaning different wavelengths bend by different amounts.

Explanation

Dispersion: refractive index depends on wavelength. The concept is dispersion, meaning different wavelengths bend by different amounts.

13. Which device relies mainly on refraction?

Loudspeaker

Microscope

Drum

Magnet

Microscopes use lenses. The concept is optical magnification by lenses, which depends on refraction.

Explanation

Microscopes use lenses. The concept is optical magnification by lenses, which depends on refraction.

14. Eyeglasses work mainly by changing how light refracts before entering the eye.

True

False

They adjust ray direction so the image forms on the retina. The concept is vision correction by refraction, shifting the focus point onto the retina.

Explanation

They adjust ray direction so the image forms on the retina. The concept is vision correction by refraction, shifting the focus point onto the retina.

15. A magnifying glass is a:

Concave lens

Convex lens

Flat mirror

Prism

Magnifying glasses are converging lenses. The concept is a convex (converging) lens, used to create a magnified view by refraction.

Explanation

Magnifying glasses are converging lenses. The concept is a convex (converging) lens, used to create a magnified view by refraction.

16. A concave (diverging) lens makes rays:

Meet at a point

Spread out

Stop

Turn into heat only

Concave lenses diverge rays. The concept is a diverging lens, causing rays to spread as if from a focal point.

Explanation

Concave lenses diverge rays. The concept is a diverging lens, causing rays to spread as if from a focal point.

17. The part of the eye that provides most refraction is the:

Retina

Iris

Cornea

Optic nerve

Cornea provides major focusing power; lens fine-tunes. The concept is corneal refraction, which contributes most of the eye’s focusing effect.

Explanation

Cornea provides major focusing power; lens fine-tunes. The concept is corneal refraction, which contributes most of the eye’s focusing effect.

18. The bending of different colours by different amounts is called ______.

Dispersion spreads colours. The concept is wavelength-dependent refraction, which produces a spectrum.

Explanation

Dispersion spreads colours. The concept is wavelength-dependent refraction, which produces a spectrum.

Submit

19. The distance from the lens to the focus is called the ______ length.

That distance is focal length. The concept is focal length, which indicates how strongly a lens converges or diverges light.

Explanation

That distance is focal length. The concept is focal length, which indicates how strongly a lens converges or diverges light.

Submit

20. Refraction changes frequency of light at a boundary.

True

False

Frequency stays the same; speed and wavelength change. The concept is constant frequency across a boundary, with changes in speed and wavelength instead.

Explanation

Frequency stays the same; speed and wavelength change. The concept is constant frequency across a boundary, with changes in speed and wavelength instead.