seegongsik
Grade 5 (age 10-11)

grade 5 Light and Lenses

Light travels in straight lines, and when it hits a smooth surface like a mirror it bounces off; the angle of incidence and the angle of reflection, both measured from the normal, are always equal. When light passes from air into water or glass, it bends at the boundary; this is called refraction. A convex lens, thicker in the middle, uses refraction to bring light together, and depending on where the object sits it either forms an upside-down image or makes things look bigger. In the first picture, change the angle of incidence and watch the angle of reflection follow. In the second, move the object closer to or farther from the lens and see how the image changes.

Reflection of light

45°
💡 Reflection of light
①Light travels in straight lines, which is why a shadow forms behind whatever blocks it.
②Light that hits a smooth surface like a mirror bounces off. This is reflection.
③The line drawn at right angles to the mirror is the normal. The angle between the normal and the incoming light is the angle of incidence, the angle with the outgoing light is the angle of reflection, and the two are always equal.
④We see objects because light reflected from them enters our eyes.

Images made by a convex lens

between F and 2F
🔍 Images made by a convex lens
①A convex lens is thicker in the middle than at the edges, so light coming in parallel is brought together at one point on the other side: the focal point.
②Put the object farther than the focal point and an upside-down image forms on the far side of the lens. At 2F it is the same size as the object; farther away it is smaller, and closer it is larger.
③Put the object inside the focal point and no image forms on the far side, but looking through the lens you see a large upright view. That is how a magnifying glass works.
④A concave lens, thinner in the middle, spreads light out, and is used in glasses for nearsighted eyes.

Refraction of light

Refraction of light
Air → water or glass: light bends at the boundary (refraction)
Refraction is also why a coin under water looks higher than it really is.
🌊 Refraction in everyday life
①A straw in a glass of water looks bent at the surface.
②The bottom of a pool or stream looks shallower than it really is.
③A rainbow forms when sunlight is refracted and reflected inside raindrops and splits into many colors.
④Sunlight passing through a prism also splits into colors from red to violet.

Summary

Light and lenses in one view
Properties of light = travels straight + reflection + refraction
Lenses use refraction to bring light together (convex) or spread it out (concave).
🎯 Key points
①Light travels straight, so it makes shadows.
②When light is reflected, the angle of incidence equals the angle of reflection.
③Light bends at the boundary when it enters a different material.
④A convex lens brings light together; magnifying glasses, cameras and the lens of the eye are convex.
⑤A concave lens spreads light out; it is used in glasses for nearsightedness.

Examples and Unit Test

Example 1
Light strikes a mirror at an angle of incidence of 30°. What is the angle of reflection?
1
When light reflects off a mirror, the angle of incidence and the angle of reflection are always equal.
2
The angle of incidence is 30°, so the angle of reflection is also 30°.
▸ 30°
Set the angle of incidence to 30° in the first picture and the angle of reflection also reads 30°.
Example 2
Why does small print look bigger through a magnifying glass?
1
A magnifying glass is a convex lens, thicker in the middle.
2
When the print is closer than the lens's focal point, the light leaving the lens reaches your eye as if it came from a bigger object, so you see a large upright view.
▸ A magnifying glass is a convex lens, and print inside its focal point looks large and upright.
In the second picture, move the object inside the focal point (F) and a large upright image appears.
Unit-test style
Which statement about light and lenses is correct?
①Light always curves as it travels.
②When light reflects off a mirror, the angle of incidence equals the angle of reflection.
③A convex lens spreads light out.
④A concave lens brings light together.
⑤A straw in water looks straight because of refraction.
▸ ② When light reflects off a mirror, the angle of incidence equals the angle of reflection.
1
When light is reflected, the angle of incidence and the angle of reflection are always equal.
2
Light travels straight (①), a convex lens brings light together (③), a concave lens spreads it out (④), and refraction makes a straw in water look bent (⑤).
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