Lenses Chapter 5

A Convex Lens is thicker at the centre than at the sides. A Concave Lens is thicker at the sides than at the centre.

The centre of a lens is called the Optic Centre. The straight line through the optic centre that is at right angles to the face of the lens is called the Axis.

A Convex Lens changes a parallel beam into a converging beam A Convex Lens changes a parallel beam into a converging beam. That’s why it’s also called a Converging lens.

A Concave Lens changes a parallel beam into a diverging beam A Concave Lens changes a parallel beam into a diverging beam. That’s why it’s also called a Diverging Lens.

A ray strikes the optic centre of a convex lens. What is its path after refraction at the lens? A ray that strikes the optic centre passes straight through the lens

A ray travelling parallel to the axis strikes the lens. What is its path after refraction at the lens? It passes through the focus at the other side of the lens.

A ray passes through the focus and strikes a convex lens. What is its path after refraction at the lens? It emerges parallel to the axis.

What is a Real Image? A Real Image is an image formed by the actual intersection of rays. A real image can be formed on a screen.

What is a Virtual Image? A Virtual Image is an image formed by the Apparent Intersection of Rays. A virtual image cannot be formed on a screen.

Light rays from Point P on the object are refracted by the lens. After refraction each of these rays passes through the same point X. The Eye sees an image of point P at X, since rays which originally were diverging from P are now diverging from X.

Formation of a Real Image in a Converging Lens

A Virtual Image in a Converging Lens

Converging Lens Formulae u is the distance from the Object to the optic centre. v is the distance from the Image to the optic centre. f is the Focal Length of the lens. If the Image is Real the formula is: If the Image is Virtual the formula is:

Magnification The Magnification m is the number of times the image is bigger (or smaller) than the object. Write down a formula relating u, v and m.

A ray of light strikes the optic centre of a diverging lens. What is its path after refraction at the lens? A ray that strikes the optic centre passes straight through the lens.

A ray that is heading for the focus of a diverging lens strikes the lens. What is its path after refraction at the lens? The ray emerges parallel to the axis

A ray travelling parallel to the axis strikes a diverging lens. What is its path after refraction at the lens? The ray emerges as if it came from the focus.

The Image in a Diverging Lens is always: Virtual, Upright (i.e. erect) and Diminished. The image is at the same side of the lens as the object.

Diverging Lens Formulae u is the distance from the Object to the optic centre. v is the distance from the Image to the optic centre. f is the Focal Length of the lens.

Power of a Lens A converging lens of shorter focal length has a greater converging effect than a lens of longer focal length. The power of the lens with the shorter focal length is larger.

Power of a Lens A diverging lens of shorter focal length has a greater diverging effect than a lens of longer focal length. The power of the lens with the shorter focal length is larger.

What type of lens has Positive Power? What type of lens has Negative Power? The power of a Converging Lens is positive (+). The power of a Diverging Lens is negative (-). What is the SI unit of Power of a Lens? The SI Unit of power of a lens is the per metre i.e. metre-1 or m-1

Two lenses of power P1 and P2 are placed in contact. Write down a formula for the Power, P, of the combination.

The Eye

What part of the eye controls the amount of light entering the eye What part of the eye controls the amount of light entering the eye? How does it do it? The Iris controls the amount of light entering the eye. It does this by making the size of the hole in its centre - the pupil - smaller in bright light and larger in dim light.

The Retina The Retina is a light-sensitive screen at the back of the eye. When light strikes the retina electrical signals are sent to the brain. The result of this is sight. When a Real Image is brought into focus on the retina, that object is seen clearly. If the image is brought to focus in front of or behind the retina the object is not seen clearly (blurred).

When a real image is brought into focus on the retina that object is seen clearly.

When a real image is brought to focus in front of or behind the retina the object is blurred.

Defects of Vision: Short Sight A Short-Sighted Person can see nearby objects clearly but cannot see distant objects clearly. The image of a distant object is formed in front of the retina with the lens at its thinnest.

How is Light Focused on the Retina? The eye contains a lens system which consists of the Cornea, which acts as a lens of fixed focal length, and an internal lens just behind the cornea that has a variable focal length.

Correction of Short Sight Short-Sight can be corrected with a Concave Lens

Defects of Vision: Long Sight A Long-Sighted person can see distant objects clearly but cannot see nearby objects clearly. The image of a nearby object is brought to a focus behind the retina even with the lens at its fattest.

Correction of Long Sight Long-Sight can be corrected with a Convex Lens