grade 7 Convex & Concave Lenses

Convex & Concave Lenses

A convex lens is thicker in the middle, converges light, and forms real or virtual images depending on object distance. Beyond the focus you get an inverted real image; inside the focus an upright magnified virtual image, as in a magnifier. A concave lens diverges light, always forms an upright reduced virtual image, and corrects myopia. Change the object distance and the convex image shifts; the concave case stays a reduced upright virtual image.

Images by a Convex Lens

Object distance20 cm

💡 Convex Lens & Light

①Convex lens: thicker at the center

②Converges light

③Object beyond focus: inverted real image

④Object inside focus: upright virtual image (magnifier)

⑤This sketch uses focal length 10 cm.

Images by a Concave Lens

🔍 Concave Lens & Light

①Concave lens: thinner at the center

②Diverges light

③Always produces an upright reduced virtual image

④Used in glasses for myopia

How Convex-Lens Images Change

Convex-Lens Image Summary

Image vs Object Position

Object position	Image type	Size	Orientation
Beyond 2F	Real	Reduced	Inverted
At 2F	Real	Same size	Inverted
Between F and 2F	Real	Enlarged	Inverted
At F	No image	-	-
Inside F	Virtual	Enlarged	Upright

Lens Equation

1f = 1a + 1b

f: focal length, a: object distance, b: image distance

Magnification

M = ba

M > 1: enlarged / M < 1: reduced

Exam Key Points

Core Summary

Convex: converging | Concave: diverging

Convex lens images vary; concave lens images are always virtual

🎯 Exam Key Points

①Convex: outside F → real (inverted); inside F → virtual (upright)

②Concave: always upright reduced virtual

③Real: forms on screen / Virtual: not on screen

④Hyperopia: convex / Myopia: concave

⑤Camera/eye: convex lens / Magnifier: virtual image inside F

Examples and Unit Test

Example 1

With a convex lens, what kind of image forms when the object is inside the focal point, and what tool uses this?

When the object is inside the focal point, a convex lens makes an upright, enlarged virtual image.

The tool that uses this is the magnifying glass.

▸ An upright, enlarged virtual image / a magnifying glass

Inside the focal point a convex lens gives an upright virtual image (magnifier); outside, an inverted real image.

Example 2

Regardless of the object position, what kind of image does a concave lens make, and where is it used?

A concave lens spreads light (diverges it) and always makes an upright, reduced virtual image.

This property is used in glasses for correcting nearsightedness.

▸ Always an upright, reduced virtual image / glasses for nearsightedness

A concave lens always makes an upright, reduced virtual image and is used to correct nearsightedness.

Unit-test style

Which statement about convex and concave lenses is correct?

①A convex lens spreads light out

②A concave lens always makes an upright, reduced virtual image

③A convex lens always makes only a real image

④A concave lens gathers light

⑤A magnifying glass uses a concave lens

▸ ② A concave lens always makes an upright, reduced virtual image

A concave lens diverges light, so regardless of object position it makes an upright, reduced virtual image.

A convex lens gathers light (①), inside the focal point it also makes a virtual image (③), a concave lens diverges light (④), and a magnifying glass uses a convex lens (⑤).

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