Why is the angle of incidence measured from the normal?

The normal is perpendicular to the surface, so it describes any incoming direction consistently.

Which way does a ray bend going into a denser medium?

It bends towards the normal, so the refraction angle is smaller.

Can a virtual image be caught on a screen?

No, it is only where backward extensions of the rays meet.

S4 Physics: Optics Practice Paper 2

2026-11-01

Tutor Prep Guide · Secondary Physics

Paper two starts with reflection and refraction angles, moves through light speed, mirror images, critical angle and Snell law, then lens images and magnification, ending with the direction mistake.

Quick answer　35 and 35 degrees, about 14.9 degrees, 1.875 x 10^8 m/s, 6 m, about 41.8 degrees, about 25.4 degrees, 12 cm at 0.5, 36 cm real at 2, n about 1.41 with 2.12 x 10^8 m/s, about 19.5 degrees.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Physics: Optics – Tutor Lesson Plan
Level	S4

02　Questions (10 items)

FoundationA ray makes 55 degrees with a plane mirror. Find the angle of incidence and of reflection.
FoundationLight enters water (n = 1.33) from air at 20 degrees. Find the angle of refraction.
FoundationA medium has refractive index 1.6. Find the speed of light inside it.
FoundationAn object stands 3 m in front of a plane mirror. Find the distance between object and image.
IntermediateLight travels from glass of refractive index 1.5 to air. Find the critical angle.
IntermediateLight hits glass of refractive index 1.5 from air at 40 degrees. Find the angle of refraction.
IntermediateA convex lens has focal length 8 cm and an object at 24 cm. Find the image distance and magnification.
ChallengeA convex lens of focal length 12 cm has an object at 18 cm. Is the image real, and what is the magnification?
ChallengeA medium has a critical angle of 45 degrees. Find its refractive index and the speed of light in it.
Error redoLight enters glass of refractive index 1.5 from air at 30 degrees. Find the refraction angle. A student answers 45 degrees; what went wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Incidence 35 degrees, reflection 35 degrees (90 – 55)	Angles are measured from the normal, not the mirror surface.
2	About 14.9 degrees	Use n1 sin t1 = n2 sin t2; entering a denser medium bends the ray towards the normal.
3	1.875 x 10^8 m/s (3 x 10^8 / 1.6)	Since n = c / v, we get v = c / n.
4	6 m (the image is 3 m behind the mirror)	A plane mirror image is symmetric about the mirror.
5	About 41.8 degrees (sin C = 1 / 1.5)	The critical angle depends only on the denser medium.
6	About 25.4 degrees	Substitute into Snell law and take the inverse sine.
7	Image at 12 cm, magnification 0.5	Use 1/f = 1/u + 1/v; magnification is v / u.
8	Image at 36 cm, real, magnification 2	With the object beyond the focal point the image is real and v is positive.
9	n is about 1.41 and v about 2.12 x 10^8 m/s	n = 1 / sin C, then v = c / n.
10	About 19.5 degrees; 45 comes from multiplying by n instead of dividing	Going into a denser medium divides by n, so the angle must shrink.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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