Ray Optics and Optical Instruments: common misconceptions

If your child says… “We measure reflection angles from the surface.”

Ask together

Which line is perpendicular to the surface here?

What’s true

On a spherical mirror, the normal passes through the centre of curvature.

If your child says… “Water makes the coin look deeper.”

Ask together

After pouring, does the coin look closer even though it has stayed still?

What’s true

Rays leaving water appear to come from a shallower position.

If your child says… “The critical angle already gives total internal reflection.”

Ask together

Which panel still has a refracted ray along the surface?

What’s true

At the critical angle, the refracted direction runs along the surface.

If your child says… “Convex glass always brings light together equally strongly.”

Ask together

What changed between these panels while the lens stayed the same?

What’s true

A lens’s focusing strength depends on its surroundings as well as its shape.

If your child says… “I add the focal lengths when lenses touch.”

Ask together

Would adding the focal lengths give the combined power written here?

What’s true

Touching thin lenses add their powers, which are reciprocals of focal lengths in metres.

If your child says… “No bending at entry means the least bending overall.”

Ask together

If entry causes no bending, must exit also cause none?

What’s true

Minimum deviation depends on the complete path through both faces.

If your child says… “The two enlargements add.”

Ask together

Does the eyepiece enlarge the original object or the objective’s enlarged image?

What’s true

The second lens acts on an image the first lens has already enlarged.

If your child says… “A wider objective magnifies more.”

Ask together

Which focal length changed when the objective became wider?

What’s true

With both focal lengths fixed, increasing objective aperture changes light collection, not angular magnification.