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. Slide the object distance to see how the convex image changes and how it differs from the concave case.

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)

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

ChartImage 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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