• Non ci sono risultati.

An interesting classroom discussion: what is the right subject of the photo?

N/A
N/A
Protected

Academic year: 2021

Condividi "An interesting classroom discussion: what is the right subject of the photo?"

Copied!
2
0
0

Testo completo

(1)

FR O N T L I N E

348 PH YS I C S ED U C AT I O N

Where teachers share ideas and teaching solutions with the wider physics teaching community: contact ped@iop.org

July 2002 CU R I O S I T Y

A regular column of enchanting curiosities, toys and crowd-pleasing demonstrations.

ing the horizontal distance apart and the length of the pendulums, students might also vary the masses, the tension in the string and the type of string.

Haima Wang

Keqiao High School, Shaoxing, Zhejiang, China

Look at the photograph: the children are my own. You are probably saying to yourself: “So what?”. An important clue is that it was taken in northern Italy on 11 August 1999 about at 1 pm. Perhaps you are still mystified. The point is that a partial solar eclipse was taking place.

Many people that day were watching the spectacle, observing the sun through protective sheets of glass or homemade optical instruments so that they could just discern a small crescent in the sky. Me too.

But while I fiddled with my rudimentary telescope— a pinhole camera made from a long cardboard tube with a hole at one end and a flimsy at the other—my 11-year-old daughter shouted: “Dad, look under that tree. What a lot of eclipses!” It was true. Large, clear images of the sun sparkled in the shadow of a leafy tree. Rather than looking up at the sky, one needed to look down at the ground.

I immediately took a photograph to capture the eclipse images on the bodies of my son and daughter.

This peculiar picture can be used with students to in-troduce geometrical optics in an unusual way. The dis-cussion can begin with the question of what is the right subject of the picture and, after revealing the little secret, some phenomenological considerations can be made. First of all, the crescent-shaped spots in the photo suggest that the usual circular bright spots within the

shadows of the trees are images of the sun, rather than images of the small gaps left by the rustling leaves. Students realize that we are surrounded by images of the sun in everyday life, e.g. those formed by sunlight filtering through shutters, Venetian blinds or holes in heavy curtains, parasols and beach-umbrellas.

Moreover, a simple experiment can be performed us-ing a sheet of paper with a hole made by a pencil (a few millimetres in diameter) and using a light source such as a chandelier holding several incandescent lamps. Holding the sheet very close to a screen, a unique spot is visible: the image of the hole. As the sheet is moved further away from the screen, the image of the hole evolves and splits into distinct spots which gradually become clearer and sharper. There are as many spots as there are bright lamps, indicating that the light sources are imaged. Their shapes resemble the filaments of the lamps and, with care, they can be focused by adjusting the sheet–screen distance. Observing the filament im-ages individually reveals that they are upside-down, re-sembling the behaviour of a pinhole camera.

All these observations suggest a classroom dis-cussion in terms of geometrical optics, based on the straight line propagation model of light for image for-mation. It is a simple example of how an activity that arises from paying attention to everyday phenomena can make the learning of physics relevant and contribute

An interesting classroom discussion:

what is the right subject of the photograph?

Time (s) x (cm) l = 20 cm l = 30 cm l = 40 cm 3 6.9 11.2 17.6 4 7.4 11.8 18.0 5 7.7 12.1 18.6 10 10.0 15.8 24.2 15 12.8 20.0 30.0 20 15.2 24.4 36.0 x l l A B

Figure 1. Schematic diagram and a demonstration

using two potatoes.

Table 1. Times recorded for various separations, x,

(2)

FR O N T L I N E

PH YS I C S ED U C AT I O N 349

Where teachers share ideas and teaching solutions with the wider physics teaching community: contact ped@iop.org

July 2002

to improving students’ powers of observation and a scientific approach to problems.

Finally, a brief his-torical note: the Arabian philosopher-scientist Abu Ali Hasan Ibn al-Haytham (Alhazen) used the pinhole camera prin-ciple to examine solar eclipses about a thousand years ago!

Federico Corni

Dipartimento di Fisica, University of Modena and Reggio Emilia, Modena, Italy email: corni@unimo.it

Look carefully to see a myriad partial eclipses.

Why or How

Why is it that a pendulum swinging on a hinge can lose amplitude and heat the hinge, but no amount of heating on the hinge will cause the pendulum to start swinging?

Guestimation

Estimate the current flowing around the equator that would cancel the Earth’s magnetic field at the poles!

Sophistry

Consider the following:

pV = k1 (Boyle’s Law)

V = k2T (Charles’ Law according to some)

P = k3T (Pressure law)

with k1, k2, k3constants.

Substitute T = p/k3from the pressure law into

Charles’ Law and it gives V = (k2/k3)p,i.e. p

proportional to V, which contradicts Boyle’s Law! (Thanks to Ken Zetie of St Paul’s School, for this.)

John Severn

Physics Teaser:

Why or How, Sophistry and Guestimation

Answers/Ideas to Teaser in May issue (where applicable)

1. Consider distribution of molecular energies. 2. Consider the fact that the nail is driven into another body!

3. (a) Treat the Earth as a uniform sphere of specific heat capacity 800 J kg−1K−1. (b) Cloud area about 107m2, about 2 km above Earth’s surface.

Riferimenti

Documenti correlati

However, if we consider natural mass scales for the supersymmetric model, which means that we should not increase the scale of the mass parameters of the model much over the TeV

Old Faithful Geyser data: (a) forward plot of minimum Mahalanobis distances from 300 random starts with 1%, 50% and 99% envelopes (two clusters are evident); (b) Bayes

At the beginning of the experiment, the calcium peroxide (CaO2) powder as a source of hydroxyl radicals (OH•) and Fenton reagent were added to the samples of wastewater.. Then, the

The researches of the seismic wave propagation velocity in the tailings massif were carried out using three measurement methods: cross-hole seismic testing (CHT) and cone penetration

Therefore, in the light of this theoretical framework and theoretical context, the main aim of this paper lies in the possibility of implementing a model based on

machines of widely different characters, of pointing out how subservience to the use of man has played that part among machines which natural selection has performed in

Cfr. Fogazzaro, Piccolo mondo antico, cit., p.. Tiziana Piras, Reminiscenze e citazioni in “Piccolo mondo antico” 207. in Milano”) 48 che ricorda Jacopo “inferraiuolato sino

A comparison of the layouts of these three documents (Table 2) shows how little they have in common in the following three categories: (1) name of the button at the foot of each