Q: What does IP Physics Notes (Upper Secondary, Year 3-4): 7) Light cover? A: Master reflection and refraction laws, critical angle behaviour, and thin-lens constructions for IP optics questions.
Quick recap -- Light travels in straight lines until a boundary bends or reflects it. Track incident and refracted angles carefully and use lens rules to predict image position, size, and orientation.
The core idea is simple: Draw the normal first, then track how light reflects or bends.
Use it as a working check: Reflection keeps equal angles. Refraction bends because speed changes. Total internal reflection needs a higher-to-lower refractive index path and an angle above the critical angle.
Then go one layer deeper: Use the mirror, optical fibre, and lens rules to practise ray diagrams that show image position, size, orientation, and whether the image is real or virtual.
Keep your practice loop tight via our IP Physics tuition hub-it links each topic here to quizzes, diagnostics, and WA-style problem sets.
For Integrated Programme students: Your current school materials, teacher instructions, and assessment scope take precedence because IP topic sequence and depth vary by school. This is an Eclat IP guide, not the O-Level / SEC G3 exam-track guide.
Eclat core: reflection, plane-mirror constructions, refraction, refractive index, total internal reflection, optical fibres, thin lenses, ray diagrams, lens calculations, and magnification follow Marcus Pang's current Chapter 7 route.
Eclat extension depth: apparent depth, lateral shift, diverging lenses, the lens formula, and linear magnification go beyond the named K323 Topic 12 outcomes but remain part of Eclat's integrated optics route.
2027 national comparison: K323 Topic 12 overlaps with reflection, refraction, refractive index, critical angle, total internal reflection, optical-fibre use, and converging-lens image construction.
Check your school: confirm whether your current assessment expects only ray construction or also algebraic lens and magnification work.
Always draw the normal at the point of incidence before marking angles.
Plane Mirror Images
Properties: virtual, upright, same size, laterally inverted, same distance behind the mirror as the object is in front.
To construct paths: draw the apparent ray from image to eye first (dashed behind the mirror), then reflect it to locate the real ray into the eye.
Plane mirror ray-path checkpoint
When a question asks how an eye sees an object in a plane mirror, start from the virtual image position, not from a guessed reflected ray.
Step
What to draw
Why it works
1
Mark the image the same perpendicular distance behind the mirror as the object is in front.
A plane mirror image is virtual and appears behind the mirror.
2
Draw a dashed straight line from the image to the eye.
This is the apparent path that the eye traces backwards.
3
Mark where that dashed line meets the mirror.
That point is where the real reflected ray must leave the mirror.
4
Join the object to that point on the mirror, then join that point to the eye.
The real light path now satisfies equal angles at the mirror.
Worked check: if two possible mirror points are shown, choose the point where the straight line from image to eye cuts the mirror. The object-to-mirror segment and mirror-to-eye segment then form the actual ray path.
Misconception check: the dashed line behind the mirror is not a real light ray. It is a construction line showing where the eye thinks the light came from.
Refraction & Refractive Index
Refraction: change in direction when light crosses media because its speed changes.
Snell's law: n1sini=n2sinr
Refractive index definition: n=vc (ratio of light speed in vacuum to medium).
An alternative form (light entering from air): n=sinrsini.
Rays bend toward the normal when entering a higher index medium (slowing down) and away when exiting to lower index (speeding up).
Refraction decision checkpoint
Before drawing or calculating, decide what has changed at the boundary.
Boundary clue
Speed change
Bend direction
What to check next
Lower refractive index to higher refractive index
Light slows down.
Towards the normal.
Refracted angle is smaller than incident angle.
Higher refractive index to lower refractive index
Light speeds up.
Away from the normal.
Refracted angle is larger than incident angle.
Higher to lower refractive index, with large incidence angle
Refraction may fail.
Ray reflects internally.
Compare the incidence angle with the critical angle.
Rectangular glass block with parallel faces
Slows on entry, speeds on exit.
Towards normal on entry, away on exit.
Emergent ray should be parallel to the incident ray, with lateral shift.
Misconception check: angles in refraction questions are measured from the normal, not from the surface. A ray that looks visually "more bent" can still have a smaller numerical angle if it is closer to the normal.
Apparent Depth & Lateral Shift
Objects under water appear shallower because emerging rays bend away; use similar triangles or Snell's law to quantify.
Glass slabs cause lateral displacement of rays while keeping them parallel.
Total Internal Reflection (TIR)
Occurs when light travels from a higher to a lower refractive index medium and the incidence angle exceeds the critical anglec.
Above c, refraction cannot satisfy Snell's law, so all light reflects internally.
Uses: optical fibres, prisms in periscopes, data transmission, and medical endoscopes.
Optical fibres carry high-bandwidth signals with low attenuation and little electromagnetic interference. Their flexibility also lets endoscopes deliver light to and return images from otherwise inaccessible parts of the body.
Worked Example: Optical Fibre Core
An optical fibre has core index n=1.48 and cladding index 1.40. The critical angle at the core-cladding boundary is
sinc=1.481.40⇒c=sin−1(1.481.40)≈72.1∘.
Any ray meeting the boundary above 72.1∘ (measured from the normal) undergoes TIR and stays in the core.
Thin Lenses
Converging (convex) lens: brings parallel rays to a focus; can form real or virtual images.
u: object distance (positive when object sits in front of lens).
v: image distance (positive for real images on the far side; negative for virtual images on the object side).
Linear magnification: m=uv=object heightimage height
∣m∣>1
Ray Construction Rules (Converging Lens)
Ray through the optical centre passes undeviated.
Ray parallel to principal axis refracts through the focus on the far side.
Ray through the near focus emerges parallel to the axis.
Image Cases for Converging Lenses
Object position
Image nature
Beyond 2f
Real, inverted, diminished between f and 2f
At 2f
Real, inverted, same size at 2f
Between f and 2f
Real, inverted, magnified beyond 2f
At f
Image at infinity
Inside f
Virtual, upright, magnified on same side
Lens answer consistency checkpoint
After using the lens equation, compare the algebraic sign with the ray-diagram picture before writing the final image description.
Calculation result
Diagram meaning
Final description to check
v>0 for a converging lens
Rays actually meet on the far side of the lens.
Real image, usually inverted.
v<0 for a converging lens
Rays diverge after the lens; their backward extensions meet on the object side.
Virtual image, upright, on the same side as the object.
∣v∣>u
Image distance is larger than object distance.
Magnified image.
∣v∣<u
Image distance is smaller than object distance.
Diminished image.
Worked check: an object is placed 8cm from a converging lens with f=12cm.
v1=121−81=−241,
so v=−24cm. The negative sign means the image is on the same side as the object. Since ∣v∣>u, the image is virtual, upright, and magnified.
Common trap: do not call every converging-lens image real. When the object is inside the focal length, the outgoing rays do not meet on the far side of the lens.
Worked Example: Lens Imaging
An object sits 18cm in front of a converging lens with f=12cm. Find the image distance and magnification.
f1=u1+v1⇒121=181+v1⇒v1=121−181=361.
So v=36cm (real image). Magnification m=uv=1836=2 -> image is inverted and twice the object height.
Diverging Lens Notes
Use the same ray rules but treat focus positions as virtual (draw them on object side).
The lens equation still applies with negative f and v for the virtual image.
Practice Quiz
Challenge yourself with Snell's law numerics, critical-angle reasoning, and thin-lens constructions in this light recap quiz.
Key Takeaways
Always mark normals and apply Snell's law or reflection laws precisely.
Critical-angle reasoning explains when light traps within a medium.
Lens calculations combine algebra (lens/magnification equations) with ray diagrams for sign and orientation checks.
Real vs virtual, magnified vs diminished outcomes stem from object position relative to f and 2f.
