DSE Physics: Optics — Tutor Lesson Plan

2026-10-31

Tutor lesson-prep guides · Secondary physics

Students can quote the lens equation but answer “describe the image” with one word. This lesson drills the full three-part answer: upright or inverted, real or virtual, magnified or diminished.

Objectives — By the end students can: use the thin lens equation to find f from u and v; state all three properties of an image from the magnification; decide from the critical angle whether total internal reflection occurs.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Reciprocals	What is 1/30 + 1/60	0.05
Sine values	What is sin 41.8 degrees roughly	about 0.667
Units	Convert 60 cm to metres	0.60 m

02 Lesson flow (60 min)

Time	Stage	Tutor does	Student does
0-8 min	Image language	Real or virtual, upright or inverted, magnified or diminished	Name all three
8-22 min	The lens equation	1/f = 1/u + 1/v plus the sign conventions	u = 30, v = 60 gives f = 20 cm
22-34 min	Magnification	m = v divided by u, and what it tells you	m = 2.0
34-46 min	Refraction limits	sin c = 1/n, dense to less dense only	c = 41.8 degrees
46-56 min	Answer form	Draw the rays, write the equation, then the numbers and units	Rewrite one answer
56-60 min	Exit check	Three questions collected and marked	Work alone

03 Teaching script (read it out)

Scene 1: Say all three properties, never one

With u = 30 cm and v = 60 cm the lens equation gives f = 20 cm and m = v/u = 2.0.
The marked answer reads “inverted, real, magnified 2.0 times”. Writing only “magnified” usually earns half marks.

Student states why a positive v means a real image (it forms on the far side and can be caught on a screen).

Scene 2: Total internal reflection needs dense to less dense

For n = 1.5, sin c = 1/1.5 = 0.667, so c = 41.8 degrees.
At 45 degrees the ray no longer refracts out; it reflects inside the glass. Light entering glass from air never does this.

Note: every angle is measured from the normal, never from the glass surface.

Scene 3: Sign conventions decide the answer

Real object u positive, real image v positive, converging lens f positive. A negative v puts the image on the same side as the object, so it is virtual and upright.
A diverging lens has negative f and always gives a diminished, upright, virtual image.

Student calculates v for the same f = 20 cm lens with u = 10 cm (minus 20 cm, virtual).

04 Question types and worked steps

Type	Example	Steps	Answer
Focal length	u = 30 cm, v = 60 cm	Write the equation, substitute, then invert	f = 20 cm
Magnification	Same data	m = v divided by u	2.0 times
Critical angle	n = 1.5	sin c = 1 divided by n	41.8 degrees
Image type	A negative v appears	Negative means same side, so virtual	Upright and virtual

Sanity check (teach this): For a real image from a converging lens both u and v exceed f. If v comes out smaller than f while the question says real, u and v have been swapped.

05 Independent practice (3 questions)

Question	Answer	What to watch
Focal length with u = 30 cm and v = 60 cm	20 cm	Equation written before substitution
Magnification for the same data	2.0	v over u, not u over v
Critical angle for glass with n = 1.5	about 41.8 degrees	Measured from the normal

06 Common mistakes and what to say

Mistake	Student writes	Say this
Magnification inverted	m written as 0.5	Magnification is image distance over object distance.
Reflection the wrong way round	total internal reflection from air into glass	It only happens from dense to less dense.
Virtual image called real	negative v still marked real	A negative v sits on the same side, so the image is virtual.
Units or properties missing	the answer is just 20	Give the unit for f and all three properties for the image.

07 Homework (paper, section and question numbers)

DSE Physics practice paper (light and lenses) — Section A, questions 6 to 8
Secondary physics chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Focal length from u = 30 and v = 60	20 cm	All three right: move to electricity
Magnification for the same data	2.0	One wrong: re-teach m = v over u
Critical angle for n = 1.5	about 41.8 degrees	Two wrong: repeat scene 2

09 Tutor FAQ

Do students need sine values by heart?

Only 30, 45 and 60 degrees; others are given or come from a calculator.

How do real and virtual images differ?

A real image is inverted, on the far side, and can be caught on a screen.

Is total internal reflection examined numerically?

Yes, usually the critical angle or a yes-or-no judgement, so sin c = 1/n matters.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.