Refraction Thin Lenses

Objectives Use ray diagrams to find the position of an image produced by a converging or diverging lens, and identify the image as real or virtual. Solve problems using the thin-lens equation. Calculate the magnification of lenses. Describe the positioning of lenses in compound microscopes and refracting telescopes.

Types of Lenses A lens is a transparent object that refracts light rays such that they converge or diverge to create an image. A lens that is thicker in the middle than it is at the rim is an example of a converging lens. A lens that is thinner in the middle than at the rim is an example of a diverging lens.

Thin Lenses Lenses are used to change the path of light rays before they enter your eyes. A lens is an object made of transparent material that has one or two curved surfaces that can refract light.

Thin Lenses

Types of Lenses Like mirrors, lenses form images, but lenses do so by refraction rather than by reflection. The front of the human eyeball acts as a lens, converging light toward the retina.

Lenses and Focal Length

Types of Lenses Ray diagrams of thin-lens systems help identify image height and location. Rules for drawing reference rays

Characteristics of Lenses Converging lenses can produce real or virtual images of real objects. The image produced by a converging lens is real and inverted when the object is outside the focal point. The image produced by a converging lens is virtual and upright when the object is inside the focal point.

Characteristics of Lenses

Converging Ray Diagrams 1.Pick a point on the top of the object and draw three incident rays traveling towards the lens.

Converging Ray Diagrams 2.Once these incident rays strike the lens, refract them according to the three rules of refraction for converging lenses. three rules of refraction

Converging Ray Diagrams 3.Mark the image of the top of the object.

Converging Ray Diagrams 4.Repeat the process for the bottom of the object.

Converging Ray Diagrams

Characteristics of Lenses Diverging lenses produce virtual images from real objects. The image created by a diverging lens is always a virtual, smaller image.

Characteristics of Lenses

The Thin-Lens Equation and Magnification The equation that relates object and image distances for a lens is call the thin-lens equation. It is derived using the assumption that the lens is very thin.

The Thin-Lens Equation and Magnification Magnification of a lens depends on object and image distances.

The Thin-Lens Equation and Magnification +- dodo Object in front of lensObject in back of lens didi Image in back of lensImage in front of lens fConverging lensDiverging lens

Sample Problem Lenses An object is placed 30.0 cm in front of a converging lens and then 12.5 cm in front of a diverging lens. Both lenses have a focal length of 10.0 cm. For both cases, find the image distance and the magnification. Describe the images.

The Eye The transparent front of the eye, called the cornea, acts like a lens. The eye also contains a crystalline lens, that further refracts light toward the light- sensitive back of the eye, called the retina.

The Eye The lens of the eye is not where the refraction of incoming light rays takes place. Most of the refraction occurs at the cornea. The cornea is the outer membrane of the eyeball which has an index of refraction of 1.38.

The Eye The image is inverted but this poses no problem Our brains have become accustomed to this and our brains properly interpret the signal as originating from a right-side-up object.

The Wonder of Accommodation The ability of the eye to accommodate is automatic. It also occurs instantaneously. Accommodation is a remarkable feat!

The Wonder of Accommodation The power of a lens is measured by opticians in a unit known as a diopter. A diopter is equal to the reciprocal of the focal length. diopters = 1/(focal length)

The Wonder of Accommodation The maximum variation in the power of the eye is called the Power of Accommodation.

Farsightedness and its Correction Farsightedness or hyperopia is the inability of the eye to focus on nearby objects. The farsighted eye has no difficulty viewing distant objects.

Farsightedness and its Correction The lens' power to refract light has diminished and the images of nearby objects are focused at a location behind the retina

Farsightedness and its Correction

Nearsightedness and its Correction Nearsightedness or myopia is the inability of the eye to focus on distant objects.

Nearsightedness and its Correction

Farsighted and Nearsighted

Combination of Thin Lenses An image formed by a lens can be used as the object for a second lens. Compound microscopes use two converging lenses. Greater magnification can be achieved by combining two or more lenses. Refracting telescopes also use two converging lenses.

Total Internal Reflection

Total Internal Reflection only takes place when both of the following two conditions are met: – the light is in the more dense medium and approaching the less dense medium. – the angle of incidence is greater than the so-called critical angle.

Total Internal Reflection Snell’s law can be used to find the critical angle. Total internal reflection occurs only if the index of refraction of the first medium is greater than the index of refraction of the second medium.

Total Internal Reflection

Atmospheric Refraction Refracted light can create a mirage. A mirage is produced by the bending of light rays in the atmosphere where there are large temperature differences between the ground and the air.

Dispersion Dispersion is the process of separating polychromatic light into its component wavelengths. White light passed through a prism produces a visible spectrum through dispersion.

Rainbows A rainbow is an excellent demonstration of the dispersion of light and one more piece of evidence that visible light is composed of a spectrum of wavelengths, each associated with a distinct color. dispersion of light visible light is composed of a spectrum of wavelengths To view a rainbow, your back must be to the sun as you look at an approximately 40 degree angle above the ground into a region of the atmosphere with suspended droplets of water or even a light mist. Each individual droplet of water acts as a tiny prism that both disperses the light and reflects it back to your eye.

Rainbows