Questions: Real and Virtual Images: Formation and Characteristics

Question 1

A photographer wants to capture an image formed by a converging lens onto a digital sensor. Which type of image is required, and why?

• A. A virtual image, because virtual images are visible to the naked eye and thus can be recorded
• B. A real image, because light actually converges at the image location, physically striking the sensor
• C. Either type works — the sensor detects brightness regardless of whether rays converge there
• D. A real image, but only if the object is beyond the focal length of the lens

Answer: (B)

A digital sensor (like film) can only record an image if light physically arrives at its surface. Real images form where refracted rays actually cross, depositing energy at that location. Virtual images form where rays only appear to diverge from — no actual light arrives there, so a sensor placed at the virtual image location detects nothing. Note that option D is partially true (objects beyond the focal length of a converging lens do produce real images) but option B correctly identifies the fundamental reason: it is about physical ray convergence, not just the lens setup.

Question 2

A student uses the thin-lens equation and finds that the image distance is −15 cm. What does the negative sign indicate about this image?

• A. The image is real, inverted, and on the opposite side of the lens from the object
• B. The image is virtual, upright, and on the same side of the lens as the object
• C. The image is real but upright — the negative sign indicates a special orientation
• D. The image is virtual and inverted — the sign only reflects the image's position, not orientation

Answer: (B)

In the standard sign convention, a negative image distance means the image is on the same side as the incoming light (the object side), which is the virtual image side for a lens. Virtual images cannot be projected onto a screen because no actual rays converge there. They are always upright (positive magnification) when formed by a single converging element used as a magnifier. Real images have positive image distance and are always inverted (negative magnification). The sign convention encodes both location and orientation consistently.

Question 3

A virtual image can be seen by the human eye even though no actual light rays converge at the image location.

• T. True
• F. False

Answer: True

True — and this surprises many students. Your eye does not require light to physically converge before reaching it; it receives diverging rays and uses its lens to focus them onto the retina. When you look at a virtual image (say, through a magnifying glass), diverging rays exit the glass and enter your eye, which automatically traces them backward to their apparent point of origin. The brain interprets this as a source at that location. A bathroom mirror produces a virtual image that you see clearly, even though placing a screen at the image location behind the mirror would show nothing.

Question 4

Real images are typically larger than the object that produced them.

• T. True
• F. False

Answer: False

False. Real images can be larger, smaller, or the same size as the object, depending on the object's distance from the lens or mirror. A camera produces a real image on its sensor that is much smaller than the scene being photographed. A projector produces a real image on a screen that is much larger than the film slide. What distinguishes real images is not their size but that they are inverted and that light physically converges at the image location. Magnification can be any negative value (inverted, real image); the magnitude can be greater or less than 1.

Question 5

Explain why a virtual image cannot be projected onto a screen, but can be seen directly by the human eye.

Model answer: A virtual image forms at the point where diverging rays appear to originate when extended backward — no actual light rays cross there. A screen placed at that location would receive no light, so nothing is projected. The human eye, however, receives the diverging rays directly and uses its own converging lens (the cornea and crystalline lens) to focus them onto the retina. The visual system automatically traces diverging rays back to their apparent source, so the brain perceives an image at the virtual location. Vision fundamentally works by interpreting diverging rays, which is why virtual images are perfectly visible — the eye finishes the job of converging the light.

This distinction is crucial for practical optics: a camera requires a real image (light must physically hit the sensor), but your eye handles virtual images with no difficulty. Magnifying glasses, eyeglasses for farsightedness, and flat mirrors all produce virtual images designed to be viewed directly by the eye.