A convex lens is thicker in the middle and brings the light passing through it together, while a concave lens is thinner in the middle and spreads light out.
What a convex lens shows depends on where the object is: beyond the focal point, an upside-down real image forms on a screen behind the lens; inside the focal point, you see an upright, enlarged virtual image when you look through the lens, which is exactly how a magnifying glass works.
A concave lens makes only an upright, smaller virtual image wherever the object is, and is used in glasses that correct nearsightedness.
Move the object from 5 cm to 35 cm and follow where the rays meet to form the image, and how the kind of image changes the moment the object crosses the focal point (10 cm).
Images Made by a Convex Lens
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Ray diagram for a convex lens (focal length 10 cm): the image forms where the rays meet
20 cm
💡 Convex Lenses and Light
①A convex lens is thicker in the middle and brings light together.
②A ray entering parallel to the axis passes through the focal point F' behind the lens, and a ray through the center of the lens is not bent; the image forms where these two rays meet.
③With the object beyond the focal point, an upside-down real image forms on a screen; the closer the object gets to the focal point, the farther away and larger the image becomes.
④With the object inside the focal point, the bent rays spread and never meet; lines traced back from them meet in front of the lens and form an upright virtual image.
Images Made by a Concave Lens
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Concave lens: lines traced back from the spreading rays meet at a small virtual image
🔍 Concave Lenses and Light
①A concave lens is thinner in the middle and spreads light out.
②A ray entering parallel to the axis spreads out as if it came from the focal point F in front of the lens.
③The spreading rays never actually meet, so only an upright, smaller virtual image forms where the traced-back lines meet.
④A nearsighted eye focuses distant objects in front of the retina; a concave lens spreads the light a little so the image lands on the retina.
How the Image Changes with Object Position
Convex lens images
How the image changes with object position (F = focal point)
Object position
Type of image
Size
Orientation
Beyond 2F
Real
Smaller
Inverted
At 2F
Real
Same size
Inverted
Between F and 2F
Real
Larger
Inverted
At F
No image
-
-
Inside F
Virtual
Larger
Upright
Lens formula (for reference)
1f = 1a + 1b
f is the focal length, a the object distance and b the image distance; the numbers in the picture come from this formula.
Magnification
M = ba
Greater than 1 means an enlarged image, less than 1 a reduced one.
Exam Key Points
Summary
convex lens: brings light together · concave lens: spreads light out
A convex lens makes real or virtual images depending on the object's position; a concave lens always makes a smaller virtual image.
🎯 Exam Points
①Convex lens: object beyond the focal point gives an inverted real image; inside it gives an upright, enlarged virtual image (magnifier).
②Concave lens: an upright, reduced virtual image wherever the object is.
③A real image is where light actually meets and can be caught on a screen; a virtual image cannot, and is seen only through the lens.
④Farsightedness is corrected with convex lenses, nearsightedness with concave lenses.
⑤Cameras and eyes use the real image of a convex lens; a magnifier uses the virtual image inside the focal point.
Examples and Unit Test
Example 1
What image does a convex lens form when the object is inside the focal point, and what tool uses this?
1
With the object inside the focal point, the rays leaving the lens spread and do not meet; traced back, they meet in front of the lens and form an upright, enlarged virtual image.
2
A magnifying glass uses this to make small print look bigger.
▸ An upright, enlarged virtual image / a magnifying glass
In the picture, set the object distance below 10 cm and a dashed image appears in front of the lens.
Example 2
What image does a concave lens form regardless of where the object is, and where is it used?
1
A concave lens spreads light, so wherever the object is, it forms an upright, smaller virtual image.
2
This is used in glasses that correct nearsightedness, where the image would otherwise form in front of the retina.
▸ Always an upright, smaller virtual image / glasses for nearsightedness
Unlike a convex lens, a concave lens can never form a real image.
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
1
A concave lens spreads light and forms an upright, reduced virtual image wherever the object is, so ② is correct.
2
A convex lens brings light together (①), it can also form a virtual image when the object is inside the focal point (③), a concave lens spreads light (④), and a magnifying glass uses a convex lens (⑤).