Romilly holds a convex lens in a sunbeam and moves a sheet of paper behind it. At one spacing the light narrows to its smallest, brightest point: 15 cm behind the lens, and nowhere else.
You have seen the same special point on the concave mirror: rays arriving parallel all meet at one point after reflecting. A convex lens has the same special point. The difference is that the light now passes through the glass instead of bouncing off it.
Sunlight arrives as a beam of parallel rays.
A convex lens is converging, so it angles those parallel rays together as they pass through.
Romilly's bright dot sits where the sun's parallel rays converge, so her lens has a focal length of 15 cm.
The focal length is a fixed property of the lens: each lens has its own.
A thin lens has a matching focal point on each side, the same distance out, because light can pass through it in either direction.
Both points are drawn as dots on the principal axis from now on.
Write the focal length in words with its value, as in: a focal length of 15 cm.
Stellan aims a convex lens at the sun and finds the smallest bright spot on a card 20 cm behind the lens. What is the lens's focal length?
The sun's rays arrive parallel to the principal axis.
Parallel rays converge at the focal point, so the bright spot sits at the focal point.
The focal length is the distance from the lens to the focal point, and that distance is 20 cm.
The lens has a focal length of 20 cm.
Two convex lenses sit side by side. Lens 1 has a focal length of 5 cm. Lens 2 has a focal length of 25 cm. Behind which lens do the sun's parallel rays converge closer to the glass?
Each lens converges the parallel rays at its own focal point.
Lens 1's focal point sits 5 cm behind it; lens 2's focal point sits 25 cm behind it.
5 cm is the shorter distance.
The rays converge closer behind lens 1, at 5 cm.
Rays arrive at a convex lens parallel to the principal axis. Where do they go after passing through?
Quill focuses the sun's rays with a convex lens and finds the smallest bright spot on a card 12 cm behind the glass. What is the lens's focal length?
Stellan says: 'A thin lens has a focal point on each side, the same distance from the glass.' Is Stellan correct?
Which statement defines the focal point of a convex lens?
Thora aims a convex lens at the sun. The smallest bright spot lands on a card 18 cm behind the lens. What is the lens's focal length?
Two convex lenses catch the same sunbeam. Lens 1 makes its bright spot 8 cm behind the glass; lens 2 makes its bright spot 30 cm behind the glass. Which statement is true?