Beijing Normal University eighth grade physics volume 2

"Lens" Common Optical Instruments PPT Courseware 4

1. Classification of lenses

1. Lens: Made of transparent material.

Classification (according to thickness)

A is thick in the middle and thin at the edges - convex lens

B is thin in the middle and thick at the edges - concave lens

C has the same thickness and thickness - flat mirror

2. Thin lens: a lens whose thickness is much smaller than the radius of the sphere.

3. Principal optical axis: the straight line passing through the centers of the two spherical surfaces of the lens.

*Note: Each lens has a main optical axis, which is drawn with dots and lines.

4. Optical center: The center point of a thin lens is called the optical center of the lens, represented by "O".

5. Demonstration experiment:

⑴ Let the convex lens face the sun, use a piece of white paper to connect it, and observe the phenomenon.

⑵ Let the concave lens face the sun, use a piece of white paper to connect it, and observe the phenomenon.

Image:

Focus: The light rays parallel to the main optical axis will converge at a point on the main optical axis after being refracted by the convex lens. This point is called the focus, represented by "F"

Focal length: The distance from the focus to the optical center O point of the convex lens is called the focal length, represented by "f".

Two focal points, two focal lengths.

Conclusion: Convex lenses have a converging effect on light.

Image:

Virtual focus: The light rays parallel to the main optical axis form divergent rays after being refracted by the concave lens. The reverse extension line of the divergent light intersects the main optical axis at a point. This point is called the virtual focus of the concave lens, represented by "F"

Focal length: The distance from the focus to the optical center O point of the convex lens is called the focal length, represented by "f".

Two virtual focus points, two focal lengths.

Conclusion: Concave lenses have a divergent effect on light.

2. The effect of lens on light

1. Converging effect of convex lens on light

Light rays parallel to the main optical axis will converge at the focus through a convex lens

B Since the optical path is reversible, the light emitted from the focus is parallel to the main optical axis after being refracted by the convex lens.

CThe propagation direction of light passing through the optical center remains unchanged

2. Concave lenses have a divergent effect on light

A A light ray parallel to the main optical axis passes through the concave lens, and the reverse extension line of the refracted light passes through the virtual focus

B Since the optical path is reversible, the light ray directed to the virtual focus is parallel to the main optical axis after being refracted by the concave lens.

C The direction of light propagation through the optical center does not change

Practice questions

1. As shown in the figure, the lens that is a convex lens is ( ) and the lens that is a concave lens is ( ).

2. For a lens, it has ( ) focal points and ( ) focal length.

3. A convex lens is thick in the middle and thin at the edges, while a concave lens is thick at the edges and thin in the middle. No matter what kind of lens, the light is always deflected to the ( ) side.

4. A convex lens has a ( ) effect on light, a concave lens has a () effect on light, and the focus of a concave lens is ( ) focus.

5. Which of the following components has a converging effect on light ( ).

A plane mirror B concave mirror C convex lens D convex mirror E concave lens

6. What is the correct statement about the light reflected by a convex lens ( )

A light passes through a convex lens and becomes parallel light

B only has a converging effect on light parallel to the main optical axis

C has a converging effect on any light

D has a converging effect on the imaging light

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