Real Images and Optical Devices

To understand which optical devices can form real images, let's first clarify what a real image is. A real image is an image that is formed by the actual intersection of light rays. It can be projected onto a screen and is formed when the light rays converge at a point after passing through an optical device.

Optical Devices that can form Real Images:
- Convex Lens (Option C)

Explanation:
A convex lens is a lens that is thicker in the middle and thinner at the edges. When light rays pass through a convex lens, they converge at a point on the other side of the lens. This point is known as the focal point, and the distance between the lens and the focal point is called the focal length.

When an object is placed in front of a convex lens, the lens refracts the light rays, causing them to converge and form an image. The image formed by a convex lens can be either real or virtual, depending on the position of the object.

- If the object is placed beyond the focal point of the convex lens, a real and inverted image is formed on the opposite side of the lens. This image can be projected onto a screen and is a real image.
- If the object is placed between the lens and the focal point, a virtual and erect image is formed on the same side of the lens. This image cannot be projected onto a screen and is a virtual image.

Optical Devices that cannot form Real Images:
- Plane Mirror (Option A)
- Convex Mirror (Option B)
- Concave Lens (Option D)

Explanation:
- Plane Mirror: A plane mirror is a flat, smooth mirror that reflects light rays without causing them to converge or diverge. It forms virtual, erect, and laterally inverted images. These images cannot be projected onto a screen, and therefore, plane mirrors do not form real images.
- Convex Mirror: A convex mirror is a mirror with a reflective surface curved outward. It reflects light rays divergently and forms virtual, erect, and diminished images. These images cannot be projected onto a screen, and therefore, convex mirrors do not form real images.
- Concave Lens: A concave lens is a lens that is thinner in the middle and thicker at the edges. When light rays pass through a concave lens, they diverge instead of converging. It forms virtual, erect, and diminished images. These images cannot be projected onto a screen, and therefore, concave lenses do not form real images.

Therefore, the correct answer is option C, a convex lens. Convex lenses can form real images when the object is placed beyond the focal point of the lens.

C - convex lens