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E. Physics: Light and Geometric Optics · Light and Geometric Optics

Light and Geometric Optics: Unit Review

A1.1 A1.2 A1.3 A1.4 A1.5 A1.6 A1.7 A1.8 A1.9 A1.10 A1.11 A1.12 A1.13 A2.1 A2.2 E1.1 E1.2 E2.1 E2.2 E2.3 E2.4 E2.5 E2.6 E3.1 E3.2 E3.3 E3.4 E3.5 E3.6 E3.7 E3.8About 40 minutes
Learning goals
  • I can describe the properties, sources and uses of light, including the electromagnetic spectrum.
  • I can predict images in plane and curved mirrors using ray diagrams and equations.
  • I can explain and calculate refraction, Snell's law and total internal reflection, and use them to explain natural phenomena.
  • I can predict images formed by converging lenses and explain how the eye and optical instruments work.

Words to know

angle of incidence
The angle between an incoming ray and the normal.
focal point (F)
The point where rays parallel to the principal axis meet (or appear to meet) after reflecting or refracting.
real image
An image formed where light rays actually meet; it can be shown on a screen.
virtual image
An image formed where light rays only appear to come from; it cannot be shown on a screen.
index of refraction (n)
The ratio of the speed of light in a vacuum to its speed in a material: n = c/v.
critical angle
The angle of incidence that gives an angle of refraction of 90°.
magnification (M)
How many times larger (or smaller) the image is than the object: M = hᵢ/hₒ = −dᵢ/dₒ.

Let's review!

In this unit you learned what light is and where it comes from, and you followed it as it reflects from plane and curved mirrors and refracts through water, glass and lenses. You drew ray diagrams, used the mirror, lens and magnification equations and Snell's law, and explained total internal reflection, mirages and rainbows. Finally, you saw how the eye and optical instruments use all of these ideas, and you evaluated technologies that help people see.

This review has a few questions from every lesson. Try each one, then press Show answer to check.

Concave mirror: object beyond CVFCobjectimage
Review figure 1 (to scale). Use the three ray rules to check: the image is smaller, inverted, between C and F, real.
Converging lens: object between 2F′ and F′FF′2F2F′objectimage
Review figure 2 (to scale). The image is larger, inverted, beyond 2F, real: this is how a projector works.

What Is Light? Waves, Energy and the Electromagnetic Spectrum

1

List these from longest to shortest wavelength: X-rays, radio waves, visible light, infrared.

2

Which carries more energy, ultraviolet or infrared? Explain.

3

Light travels 1.50 × 10¹¹ m from the Sun to Earth at 3.00 × 10⁸ m/s. How long does it take?

How Light Is Produced: Incandescence, Luminescence and LEDs

1

Name how each produces light: a glow stick, a firefly, a toaster element.

2

How is phosphorescence different from fluorescence?

3

Why are LEDs more efficient than incandescent bulbs?

The Ray Model and the Laws of Reflection

1

A ray strikes a plane mirror with an angle of incidence of 35°. What is the angle of reflection?

2

A ray makes an angle of 25° with the mirror's surface. Find the angle of incidence.

3

Why can you see your reflection in a mirror but not in a sheet of paper?

Images in Plane Mirrors

1

Describe a plane mirror image using SALT.

2

You stand 1.5 m in front of a plane mirror. How far are you from your image?

3

Why is a plane mirror image called virtual?

Curved Mirrors: Concave, Convex and the Focal Point

1

A concave mirror has a radius of curvature of 40 cm. What is its focal length?

2

Where do rays parallel to the principal axis go after reflecting from a concave mirror?

3

Which type of mirror is used for store security and car passenger-side mirrors? Why?

Ray Diagrams for Concave Mirrors

1

An object is beyond C of a concave mirror. Describe the image.

2

Where should you place an object in front of a concave mirror to get an upright, enlarged image?

3

What happens when the object is exactly at F?

Ray Diagrams for Convex Mirrors and Their Uses

1

Describe the image in a convex mirror.

2

Why do passenger-side mirrors say "Objects in mirror are closer than they appear"?

3

Which way does the reflected ray go when a ray aimed at F (behind a convex mirror) hits the mirror?

The Mirror and Magnification Equations

1

A concave mirror has f = 10 cm. An object is 30 cm away. Find dᵢ and M.

2

A convex mirror has f = −12 cm. An object is 24 cm away. Find dᵢ and M.

3

What does a negative image distance tell you?

Refraction and the Index of Refraction

1

The index of refraction of diamond is 2.42. Find the speed of light in diamond.

2

Light passes from air into water at an angle. Which way does it bend, and why?

3

Light in a material travels at 2.00 × 10⁸ m/s. Find n.

Investigating Refraction: Snell's Law

1

Light goes from air into glass (n = 1.50) with θi = 30°. Find θR.

2

Light goes from water (n = 1.33) into air with θi = 30°. Find θR.

3

In a refraction experiment, why is it better to use many angles than one?

Partial Reflection and Total Internal Reflection

1

Find the critical angle for water (n = 1.33) to air.

2

State the two conditions for total internal reflection.

3

Name two technologies that use total internal reflection.

Light in Nature: Apparent Depth, Mirages, Shimmering and Rainbows

1

Why does a fish look closer to the surface than it really is?

2

Where must the Sun be for you to see a rainbow?

3

Explain the "wet road" mirage on a hot day.

Lenses and Ray Diagrams for Converging Lenses

1

An object is beyond 2F′ of a converging lens. Describe the image and name a device that uses this.

2

Where must an object be for a converging lens to act as a magnifying glass?

3

Describe the path of a ray that passes through the optical centre of a thin lens.

The Thin Lens Equation: Predicting and Testing Images

1

A converging lens has f = 10 cm and an object 15 cm away. Find dᵢ and M.

2

The same lens has an object 5.0 cm away. Find dᵢ and M.

3

A 2.0 cm object gives M = −3.0. Find the image height.

The Eye and Technologies for Seeing Better

1

Which type of lens corrects nearsightedness?

2

A lens has f = −0.50 m. Find its power.

3

How does laser eye surgery correct vision?

Optical Instruments: Cameras, Telescopes, Microscopes and More

1

A telescope has fₒ = 1000 mm and fₑ = 25 mm. Find the magnification.

2

A microscope has a 10× eyepiece and a 10× objective. Find the total magnification.

3

What does a retroreflector do, and where is one used?

Just for fun

Why did the student bring a mirror to the unit review?

To reflect on everything she had learned. And when anything looked blurry, she knew exactly how to refocus.

How to use this review

  1. Do the questions for each lesson on paper first, with a ruler for any diagrams.
  2. Press Show answer and mark yourself honestly.
  3. If you miss more than one question in a set, reread that lesson's examples and redo its "Your turn" questions.
  4. Finish with the quiz below. Aim for 8 out of 10 or better.
  5. Keep a list of formulas: n = c/v, n₁ sin θ₁ = n₂ sin θ₂, 1/f = 1/dₒ + 1/dᵢ, M = hᵢ/hₒ = −dᵢ/dₒ, P = 1/f.
Check your understanding

Quiz

1.

Which type of EM radiation has the highest frequency?

2.

A ray hits a plane mirror at 40° to the normal. The angle of reflection is:

3.

The image in a plane mirror is:

4.

A concave mirror has f = 15 cm and an object at 30 cm (at C). The image is at:

5.

A convex mirror always forms an image that is:

6.

The speed of light in glass with n = 1.50 is:

7.

Light travels from glass into air. Total internal reflection can happen when the angle of incidence is:

8.

A converging lens (f = 20 cm) has an object 10 cm away. The image is:

9.

Farsightedness is corrected with:

10.

A rainbow forms mainly because:

For parents and teachers

How to use the review results:

  • Ordering the EM spectrum or linking frequency and energy → revisit What Is Light? Waves, Energy and the Electromagnetic Spectrum.
  • Mixing up fluorescence, phosphorescence and the other ways of producing light → revisit How Light Is Produced: Incandescence, Luminescence and LEDs.
  • Measuring angles from the mirror instead of the normal → revisit The Ray Model and the Laws of Reflection.
  • Plane-mirror image distance or describing images with SALT → revisit Images in Plane Mirrors.
  • Confusing concave and convex, or f and R → revisit Curved Mirrors: Concave, Convex and the Focal Point.
  • Ray diagrams for concave mirrors (wrong ray rules, wrong image location) → revisit Ray Diagrams for Concave Mirrors.
  • Convex-mirror diagrams or uses → revisit Ray Diagrams for Convex Mirrors and Their Uses.
  • Sign errors with the mirror equation (convex f must be negative) → revisit The Mirror and Magnification Equations.
  • Speed of light in a material, or which way light bends → revisit Refraction and the Index of Refraction.
  • Snell's law calculations (calculator in degree mode!) → revisit Investigating Refraction: Snell's Law.
  • Critical angle or the conditions for total internal reflection → revisit Partial Reflection and Total Internal Reflection.
  • Explaining apparent depth, mirages or rainbows → revisit Light in Nature: Apparent Depth, Mirages, Shimmering and Rainbows.
  • Lens ray diagrams or matching object positions to devices → revisit Lenses and Ray Diagrams for Converging Lenses.
  • Thin lens equation and magnification, especially virtual images → revisit The Thin Lens Equation: Predicting and Testing Images.
  • Vision problems, corrective lenses or dioptres → revisit The Eye and Technologies for Seeing Better.
  • Telescope or microscope magnification, or how instruments work → revisit Optical Instruments: Cameras, Telescopes, Microscopes and More.

Tip: Most mistakes in this unit come from three habits: measuring angles from the surface instead of the normal, forgetting the sign of f or dᵢ, and drawing ray diagrams without a ruler. Encourage your teen to draw every diagram carefully with a sharp pencil and a ruler.