Reflection, Refraction & Optics — Practice · Grade 12 Science

Chapter: Waves, Sound & Optics

What this practice covers

By the end of Reflection, Refraction & Optics you should be able to:

11 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: reflection, normal, refraction, index of refraction (n), total internal reflection, focal length (f), real vs. virtual image.

Problem 1

In the law of reflection, the angle of incidence and angle of reflection are measured from…
  1. the horizon
  2. the edge of the mirror
  3. the surface itself
  4. the normal (the perpendicular to the surface)
Answer and explanation

the normal (the perpendicular to the surface)

All angles in optics are measured from the normal, the line perpendicular to the surface at the point of contact.

Problem 2

Light travels at 2 × 10⁸ m/s inside a block of glass. Using c = 3 × 10⁸ m/s, find the glass's index of refraction.
Answer and explanation

1.5

n = c ÷ v = (3 × 10⁸) ÷ (2 × 10⁸) = 1.5.

Problem 3

When light passes from air into glass (a higher-index medium), it…
  1. bends toward the normal and slows down
  2. bends toward the normal and speeds up
  3. bends away from the normal and speeds up
  4. passes straight through with no change
Answer and explanation

bends toward the normal and slows down

Entering a denser, higher-index medium, light slows down and bends toward the normal.

Problem 4

An object is placed 30 cm from a converging lens with a focal length of 15 cm. Find the image distance.
Answer and explanation

30 cm

1 ÷ dᵢ = 1 ÷ f − 1 ÷ dₒ = 1 ÷ 15 − 1 ÷ 30 = 2 ÷ 30 − 1 ÷ 30 = 1 ÷ 30, so dᵢ = 30 cm.

Problem 5

Explain the difference between a real image and a virtual image.
Answer and explanation

A real image forms where light rays actually converge, so it can be projected onto a screen (like the image on a camera sensor or a movie screen); it is usually inverted. A virtual image forms where rays only appear to come from — they never actually meet there — so it cannot be projected; it is upright, like the enlarged image seen through a magnifying glass or your reflection in a flat mirror.

Problem 6

An object 2 cm tall is placed 10 cm from a lens, and its real image forms 30 cm away on the other side. How tall is the image? (magnification magnitude = image distance ÷ object distance)
Answer and explanation

6 cm

Magnification magnitude = dᵢ ÷ dₒ = 30 ÷ 10 = 3, so the image height = 3 × 2 cm = 6 cm.

Problem 7

An object is 20 cm from a lens with f=10 cm. Find image distance. Show the reasoning, not just the conclusion.
Hints

State the sign convention; a negative image distance has a different interpretation.

Answer and explanation

1/d_i=1/10−1/20=1/20; d_i=20 cm.

Problem 8

Why does a ray bending toward the normal not by itself give a new frequency?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Refraction changes propagation speed and wavelength while frequency is continuous across a stationary boundary.

Problem 9

Lens signs describe location and orientationLens signs describe location and orientationThin converging lensValueConventionFocal length f+10 cmConverging positiveObject distance do+20 cmReal object sideImage distance di+20 cm1/di=1/f−1/doMagnification m−1m=−di/doA negative magnification indicates inversion.
Calculated thin-lens cases; paraxial rays and negligible lens thickness are assumed.
Interpret the supplied visual. What happens for this lens with a real object at do=5 cm?
Answer and explanation

di=−10 cm and m=+2: an upright enlarged virtual image on the object side.

In the stated thin-lens convention, 1/f=1/do+1/di, with f positive for a converging lens, do positive for a real object and di positive for a real image on the far side. Magnification m=−di/do gives image height divided by object height. For f=10 cm and do=20 cm, di=20 cm and m=−1: the real image is inverted and equal in height. State the convention rather than interpreting every negative sign as an error.

di=−10 cm and m=+2: an upright enlarged virtual image on the object side.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

A thin converging lens has f=12 cm and a real object at 36 cm. Find image distance and magnification; describe the image.
Answer and explanation

1/di=1/12−1/36=1/18, so di=18 cm and m=−18/36=−0.50. It is real, inverted and half the object height in this paraxial model.

1/di=1/12−1/36=1/18, so di=18 cm and m=−18/36=−0.50.

It is real, inverted and half the object height in this paraxial model.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 11

OpenSciEd room diagram for tracing light from a lit object through or off a separating surface to an observer; ray paths are left for the learner to complete.
The two-room source template leaves ray paths unfinished. Use the local completed ray model to reason about source, boundary and observer. Two-room light-path model · OpenSciEd · 6.1 Light & Matter, Lesson 2, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
What needs to be drawn for an observer to receive light from the source by reflection or transmission? State which direction the light travels and how reflection/refraction angles are defined.
Answer and explanation

Draw source-to-surface rays, then reflected or transmitted paths that actually reach the observer. Light enters the eye from the path; the eye does not emit rays that create sight. Angles are measured from the boundary normal, reflection uses equal angles and transmitted direction requires the indices and geometry.

Read source labels, scope, axes and intentional omissions.

Use the complete local explanation to draw a justified mechanism or calculate a supported quantity.

State what is observed, what is assumed and what evidence remains needed.

Response criteria

  • Interpret the source labels, units and scope accurately.
  • Explain the relevant mechanism with a labeled model or supported calculation.
  • Identify a meaningful assumption, uncertainty or missing comparison.

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