Light and Optics
Trace what light does at a boundary, predict image properties, and explain optical observations without guessing from memorized labels.
Light and Optics
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Measure from the normal, then decide whether rays meet or only appear to meet
For reflection, the angle of incidence equals the angle of reflection. Both angles are measured from the normal—the imaginary line perpendicular to the surface.
Refraction occurs because light changes speed between media. Entering a higher-index medium usually bends a ray toward the normal; entering a lower-index medium bends it away from the normal. Frequency remains fixed, so a speed change produces a wavelength change.
Real versus virtual images
A real image forms where rays actually converge and can be projected onto a screen. A virtual image forms where diverging rays only appear to originate and cannot be caught on a screen.
Converging versus diverging devices
Concave mirrors and convex lenses can converge rays. Convex mirrors and concave lenses diverge rays and usually form upright reduced virtual images.
Use SALT to organize image questions
S — Size: larger, smaller, or same?
A — Attitude: upright or inverted?
L — Location: in front/behind a mirror or on which side of a lens?
T — Type: real or virtual?
Then check the physical clue: if the image appears on a screen, it must be real.
Why it works
SALT separates four properties students often mix together and lets experimental evidence—especially projection onto a screen—eliminate impossible choices.
Five forms you should recognize
Setup: A candle is 20 cm in front of a plane mirror.
Result: Its virtual upright image appears 20 cm behind the mirror, the same size as the candle.
Setup: A ray enters glass obliquely from air.
Result: It slows and bends toward the normal; frequency stays constant while wavelength decreases.
Setup: An object is beyond twice the focal length of a convex lens.
Result: A real, inverted, reduced image forms between F and 2F on the other side.
Setup: A face is closer to a concave mirror than its focal point.
Result: The reflected rays diverge, but their backward extensions form an upright magnified virtual image behind the mirror.
Setup: Light inside glass meets the glass–air boundary at an angle greater than the critical angle.
Result: No refracted ray escapes; the light reflects back into the glass.
Check before you commit
- Measuring angles from the surface instead of the normal
- Saying frequency changes when light crosses a boundary
- Assuming every magnified image is real
- Confusing convex mirrors with convex lenses
- Calling ordinary Moon-like color changes dispersion
- Using total internal reflection for light entering a denser medium
Do you need the lesson-or just practice?
One original question in each form recommends your next step. It does not yet verify mastery.
Work at the level you need.
Foundations
Build the core procedure with immediate explanations.
Core Practice
Use mixed forms with less scaffolding.
UPCAT-Style Transfer
Apply the competency in unfamiliar representations.
Ready to verify this competency?
A score of 5/5 verifies mastery. An unsuccessful attempt loads a different five-form bank.
Light and Optics FAQ
Why does a straw look bent in water?
Light changes direction as it leaves water for air, so the submerged part appears displaced.
Why can a virtual image be seen but not projected?
Rays entering the eye behave as though they came from the image location, but they do not physically converge there.
Why do objects have color?
Their surfaces selectively reflect or transmit some visible wavelengths and absorb others.
Continue your science review.
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